Share Extension: Can be crashed with weird HTML

[nheagy]

I believe this may be the cause of #10263

Steps to reproduce the crash:

To crash the app:

• Go to Notes.app
• Type in this will <crash>crash</crash> the share extension
• Share to WordPress using the share extension
• Nothing appears (the extension crashes).

Why this happens

Currently, plain text sent to the share extension is treated as HTML, and wrapped in <blockquote>. You can see this by sharing "my <em>note</em>" from Notes.app. It will appear in our share extension as:

my note

So when sharing the crashing string above, the Aztec instance within the share extension doesn't know what to do with the <crash> tag, and crashes.

There are two components to this issue:

1. Incoming plain text is treated as HTML, but instead its HTML entities should be escaped
2. Aztec within the share extension is unprepared for weird HTML

The latter of which will be important if the share extension ever intends to support HTML import.