Manually pause/start background sync and notifications

**Is your feature request related to a problem? Please describe.**
I'm using Element for work, but when I'm using my computer, I use the html client and I do not need the app anymore. This is also the case when I finish my day, I do not want to have notifications anymore; I do not want to drain battery for fetching messages that I won't read, and to be bored by notifications that I'll see the next day.

**Describe the solution you'd like**
I'd love a kind of toggle somewhere (probably in the main menu) to be able to quickly start / stop the sync.

**Describe alternatives you've considered**
I considered uninstalling the app each time I do not need it anymore :( but it's obviously not a solution; no to mention that I would adjust energy saving settings to say to my phone to not kill the app when it's in background and the phone is locked.

I noticed that there's already [another Feature Request](https://github.com/vector-im/element-android/issues/1860) for a similar idea.

Thanks !