Move purple-specific preferences to sub-page

We have a sub-page for SMS/MMS options, so I think it'd make sense to move other protocol-specific options to their own sub-page near the top, and only have general-purpose options on the first level.

What I'm imagining is a section at the end of the preferences named something like "Protocols", with rows like "SMS and MMS", "XMPP" (which would have purple stuff), and in the future maybe "Matrix". Each of these would then open a sub-view, similar to the SMS/MMS one.

Then the structure of the preferences page would look something like this:

# Accounts
- Add Account

# Privacy
- Message Receipts
- Typing Notifications

# Edtior
- Graphical Emoji
- Send Messages with Return

# Protocols
- SMS and MMS
- XMPP