[UI] Show loading indicator when loading messages instead of "No messages"

[MexHigh]

Is your feature request related to a problem? Please describe.
I have around 10 applications in Gotify, all of them receiving messages regularly. When I want to check them in the UI, I usually do this app by app and not via the "All messages" tab. When switching between application tabs, there is a short amount of time, where the messages are being fetched and the messages panel is empty saying "No messages".

If I want to quickly take a look at all application tabs, sometimes I'm not waiting long enough for the messages to come in and then assume that there are none, because there was no loading indicator.

Describe the solution you'd like
There should be some kind of loading indicator (e.g. a spinner) when opening an application tab the first time. This should only introduce changes in the ui, not the backend code.

Describe alternatives you've considered
None.

Additional context
Not required, I think.

[jmattheis]

Sounds like a useful addition 👍

[MexHigh]

If I find some time, I'll try to implement this next week myself and open a PR

[MexHigh]

So, after a wrongly pushed branch it should work now, feel free to review #660