Add command line application to sanitize HTML files

This can be used to test what HTML messages will look like after K-9 Mail's sanitization process.

cli/html-cleaner-cli/README.md

@@ -0,0 +1,17 @@
+```text
+Usage: html-cleaner [OPTIONS] INPUT [OUTPUT]
+
+  A tool that modifies HTML to only keep allowed elements and attributes the
+  same way that K-9 Mail does.
+
+Options:
+  -h, --help  Show this message and exit
+
+Arguments:
+  INPUT   HTML input file (needs to be UTF-8 encoded)
+  OUTPUT  Output file
+```
+
+You can run this tool using the [html-cleaner](../../html-cleaner) script in the root directory of this repository. 
+It will compile the application and then run it using the given arguments. This allows you to make modifications to the
+[HTML cleaning code](../../app/html-cleaner/src/main/java/app/k9mail/html/cleaner) and test the changes right away.

cli/html-cleaner-cli/build.gradle

@@ -0,0 +1,20 @@
+apply plugin: 'org.jetbrains.kotlin.jvm'
+apply plugin: 'application'
+
+version 'unspecified'
+
+application {
+    mainClass = "app.k9mail.cli.html.cleaner.MainKt"
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    implementation project(':app:html-cleaner')
+
+    implementation "org.jetbrains.kotlin:kotlin-stdlib"
+    implementation "com.github.ajalt.clikt:clikt:3.4.0"
+    implementation "com.squareup.okio:okio:${versions.okio}"
+}

cli/html-cleaner-cli/src/main/kotlin/Main.kt

@@ -0,0 +1,55 @@
+package app.k9mail.cli.html.cleaner
+
+import app.k9mail.html.cleaner.HtmlHeadProvider
+import app.k9mail.html.cleaner.HtmlProcessor
+import com.github.ajalt.clikt.core.CliktCommand
+import com.github.ajalt.clikt.parameters.arguments.argument
+import com.github.ajalt.clikt.parameters.arguments.optional
+import com.github.ajalt.clikt.parameters.types.file
+import com.github.ajalt.clikt.parameters.types.inputStream
+import java.io.File
+import okio.buffer
+import okio.sink
+import okio.source
+
+@Suppress("MemberVisibilityCanBePrivate")
+class HtmlCleaner : CliktCommand(
+    help = "A tool that modifies HTML to only keep allowed elements and attributes the same way that K-9 Mail does."
+) {
+    val input by argument(help = "HTML input file (needs to be UTF-8 encoded)")
+        .inputStream()
+
+    val output by argument(help = "Output file")
+        .file(mustExist = false, canBeDir = false)
+        .optional()
+
+    override fun run() {
+        val html = readInput()
+        val processedHtml = cleanHtml(html)
+        writeOutput(processedHtml)
+    }
+
+    private fun readInput(): String {
+        return input.source().buffer().use { it.readUtf8() }
+    }
+
+    private fun cleanHtml(html: String): String {
+        val htmlProcessor = HtmlProcessor(object : HtmlHeadProvider {
+            override val headHtml = """<meta name="viewport" content="width=device-width"/>"""
+        })
+
+        return htmlProcessor.processForDisplay(html)
+    }
+
+    private fun writeOutput(data: String) {
+        output?.writeOutput(data) ?: echo(data)
+    }
+
+    private fun File.writeOutput(data: String) {
+        sink().buffer().use {
+            it.writeUtf8(data)
+        }
+    }
+}
+
+fun main(args: Array<String>) = HtmlCleaner().main(args)

html-cleaner

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+./gradlew --quiet ":cli:html-cleaner-cli:installDist" < /dev/null && ./cli/html-cleaner-cli/build/install/html-cleaner-cli/bin/html-cleaner-cli "$@"

settings.gradle

@@ -25,3 +25,4 @@ include ':backend:webdav'
 include ':backend:jmap'
 include ':backend:demo'
 include ':plugins:openpgp-api-lib:openpgp-api'
+include ':cli:html-cleaner-cli'