Add ability to to parse and handle pasted HTML content

[Half-Shot]

• Basic functionality
• Works on Web
• Works on Android / iOS
• Tests

[Half-Shot]

I started working on this, but I found that the insertion method I'm using is inserting extra elements into the tree. I feel like this needs better stripping semantics when pasting HTML.

Additionally, this uses the same parser we use for Matrix-HTML, which means it blows up on anything unexpected.

[Half-Shot]

I think @langleyd has this covered.