add ability to manage plugins through the web UI

**Is your feature request related to a problem? Please describe.**
It is frustrating to have to upload plugin files on the sever to install or update them. This becomes even more of a problem when developing plugins, were you have to do this often.

**Describe the solution you'd like**
It would be great to have an upload button on the plugins page to upload the plugin file to the server.
A delete and update button on the individual plugin page to make changes to existing plugins.
We would also need a way to restart the Gotify service through the UI to apply the changes.

**Additional context**
I can foresee an issue with breaking the Gotify service from a broken or incompatible plugin. This is already an issue when manually uploading a plugin file. Currently it is not that bad since you will already be changing files at the server level. I think there may be problems that occur when opening up the plugins feature to more users who may not readily access the service's files.

This is something I would be glad to implement if others would find value in it.