Fetch channels

[makmu]

This is work in progress, so please don't merge yet.

These are some of the things we should do before merging this feature into master:

• Remove channel id from settings. To be able to do that we have to refactor the "post as reply"-feature a bit because currently there is the channel id needed to post replies.
• reduce settings dialog after channel id has been removed from settings
• split channellist and channel class into separate files
• fix redundancy of Channel class in OutlookMatters/Mattermost/Session/Channels.cs and OutlookMatters/Mattermost/ChannelList.cs
• move constant for subscribed channels from MailItemContextMenuEntry to some place within the mattermost namespace (because - semantically - the constant is part of the REST API)
• restructure test assembly to mirror the production assembly

[codecov-io]

Current coverage is 58.58%

Merging #35 into master will increase coverage by +2.42% as of 73c7696

@@            master     #35   diff @@
======================================
  Files           34      35     +1
  Stmts          641     693    +52
  Branches        25      28     +3
  Methods                          
======================================
+ Hit            360     406    +46
  Partial          1       1       
- Missed         280     286     +6

Review entire Coverage Diff as of 73c7696

Powered by Codecov. Updated on successful CI builds.