Move llavatarname.* to llmessage/ from llcommon/
Update/Modernize LLAvatarName and LLAvatarNameCache and _EVERYTHING_ they touch

Fairly certain this adds the feature in which the user may click the online notification to open a chat with the user coming online.

Translators may want to look at the xml changes of this change to see what to update

Removes old LegacyMultiAttachmentSupport patch
Moves LLAvatarName-centric functions into LLAvatarName from elsewhere.
Adds boost::signals2::connections to name polling places.
Removes more old icky static callbacks
Merge from Alchemy, credit where due~
Update notifications.xml to have a bunch of the tags from upstream to ease diffing
Update and Sync Mutelist with upstream alchemy

Satisfies the feature request for mute toggle to display mute status
Adds dynamic mute/unmute option to P2P IM dropdown.

Translators should look into the changes made in this commit for:
floater_chat_history.xml, floater_instant_message.xml, floater_instant_message_concisebuttons.xml
panel_avatar.xml has been taken care of for spanish and french, although they should be looked into more closely by the translators.

Cleanup and Sync in related places~
Removal of old static callbacks.
Moves God Names logic into LFSimFeatureHandler
Adds a custom function LLMuteList::hasMute() for quickly checking if a mute is in the internal set.