Add loading indicator / skeleton loading

[thecodeah]

Problem

There is currently no indication that anything is being fetched when the application is started. This may in some cases take over 10 seconds.

Proposal

Add a loading indicator or skeleton loading for when the inbox is being fetched and when the content of an email is being fetched.

Prior Art (Optional)

No response

[teamcons]

I need to look on how other apps show there is something loading. This feels quite important to me - theres been instances where i wondered what the heck Mail was doing and whether it was doing anything at all