Channel sidebar should push content over, rather than slide on top to follow native iOS design

[esethna]

Expected (See Mail and Slack sidebars): Post list content is pushed right when the sidebar opens. The sidebar is underneath (lower vertical height) the post list - see Mail or Slack apps for examples of this native behaviour. In addition, tapping, holding and sliding your finger to the right anywhere on the post list content should start opening the sidebar.

Observed: The sidebar pulls out over top of the content when swiping only from the edge of the screen. See video in ticket

Note that on Android the sidebar should behave as it does now (does not push the content over). This is only a change for iOS to behave more natively.

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JIRA: https://mattermost.atlassian.net/browse/MM-18530

[nick-brady]

Hey there! I started this today and progress was pretty smooth. I still need to clean up the code and do some testing, but I should be able to make a PR soon.

One note, about this:

In addition, tapping, holding and sliding your finger to the right anywhere on the post list content should start opening the sidebar.

the "swipe anywhere" is really a separate task, and I found this issue here which covers it. My PR addresses the first part of this ticket, which would be changing iOS behavior to have the content move over the sidebar.

[migbot]

Thanks @nick-brady!