Hashtags: Protect HTML tags from being broken

[akirk]

If your post has HTML that contains something that resembles a hashtag, the ActivityPub plugin currently breaks that tag by replacing new HTML tags into the other tag.

Specifically, this HTML:

<span style="color: #ccc">Only friends</span>

will be transformed to this:

<span style="color: <a rel="tag" class="u-tag u-category" href="https://alex.kirk.at/tag/ccc/">#ccc</a>;&#8221;>Only friends</span>

which visually appears like this:

This PR adds protection of the tag before the replacing operation happens and then replaces them back in, resulting in unmodified tags inside HTML:

[pfefferle]

Awesome! Thanks a lot!