Rename `Channels` and `Posts` to `Discussions`

[vikasrohit]

Rename Channels and Posts to Discussions to keep things consistent.

Continued from #2301

Aha! Link: https://topcoder.aha.io/features/TCCONNECT-387

[vikasrohit]

@vic-topcoder @acshields we already have the label Discussions for a tab which is visible to only very very very old projects and I have seen active projects (e.g. https://connect.topcoder.com/projects/1288) with that tab enabled. If you think this change does not cause confusions to those project members, we are good with the requirements here.

[vic-georgiev]

This is in a sense update of the feature we had :) Those projects ideally would be upgraded to the new system.

[vikasrohit]

Agreed @vic-topcoder. However, as per my understanding after reading the mangers (for those legacy projects) views, these projects are not going away very soon. Also, Discussions tab is a legacy thing that is there, which we have hidden for all new projects but they would remain there for older ones to keep there records. So, if we are okay with renaming Channels to Discussions even for such projects, we are good here.

[RishiRajSahu]

fixed with 8110363