Before this commit chat was applying a fixed height on everything under the `/chat` route. It's only really needed on the channel page with the composer at the bottom of the page.
This commits makes the following changes:
- moves height limitation from `#main-outlet-wrapper` to `.chat-channel`
- makes browse channel page and members list page full height and rely on main document scrollbar
- adds height computation for draft header and direct message creator block to ensure the height is correct when creating a draft channel
- makes chat index full height to rely on the browser scrollbar. As a result the <kbd> + </kbd> button used on mobile to create a direct message as been moved out of `<ChannelsList>` into the chat index template
- sidebar height was relying on chat setting a max height, as a result the height computation of sidebar has been changed to work correctly, especially with an opened keyboard on mobile or ipad
- Raises the scroll distance to 250px instead of 100px to show the arrow down button
- Always have a margin on drawer when showing channel list, removes this margin when the scrollbar is apparent
- Makes all scrollbar used in chat look the same through the chat-scrollbar mixin
- Ensures hover state is not persistent on channel row in mobile
- Makes the channel row full width on mobile
- Note this is also tweaking the UI a little bit as we are now using links/buttons in the header as needed
- It disables the find ideal channel in drawer mode, if loading `/chat` in drawer mode it will either reopen at the last position or just stay on index
