Ability to add tags alongside/instead of setting tags

[gBillal]

Originally discussed in #1727

We want to add tags to existing devices. Currently there's only setting tags which requires typing all the existing tags.

A use case
Imagine having 10 or 100 devices with a set of tags (there's a unique tag per device), and you want to add a common tag, in this case, you need to add the tag to the devices one by one otherwise you risk resetting all of them.

It would be great if there's a nice way to delete a tag too :)