Restructure meetings to handle >2 participants

[darmai]

Remove the current assumption from the code that only 2 users will be involved in a meeting. High level steps:

1) store meeting participants in a meeting_participant table associating users to a meeting 2) remove invitee_id and user_id field from meeting table

[darmai]

Meeting participants are now stored in a meeting_participants table. This table also stores whether or not the participant created the meeting. Code updated to reflect this change so that there is no longer and reliance on the invitee_id and user_id fields of the meeting table. The commits implementing these changes are:

168 #170 #171 #172 #173

[darmai]

Removed any references to users table within meetings. All associations between meetings and users now stored in meeting_participants

Migrations created #174