Hashtag: Ignore tags within HTML attributes (like hex colors)

[obenland]

Fixes #955.

Proposed changes:

• Strips tags before searching for hashtags to convert to tags.
• Adds unit test for various scenarios where hashtags should and shouldn't be converted.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Create a new Post, use the code editor.
• Enter an HTML tag with a css hex color code in its style. e.g.: <span style="color: #ffffff"> (Important: preceding whitespace)
• Save the post
• Make sure the post doesn't have a tag called ffffff.

[pfefferle]

this is so simple and smart!