Escape pasted text in compose area

[dbrgn]

Sanitizing text would cause some non-HTML text to disappear (see #86). Instead, we escape HTML, so that it looks exactly like pasted.

The applyFilters helper function has been removed, it's unclear what it does. Being explicit is better in this case.

Fixes #86.

[dbrgn]

@sillych I'm not sure whether we should remove the htmlToAsciiMarkup filter. What do you think happens more often, people pasting rich formatted text (e.g. bold) from a website or an e-mail and wanting it to be formatted, or people sharing HTML markup with coworkers? In the latter case, they'd want the HTML code to remain unchanged.