Notify user if an update is available

[ninjadev64]

Description

Shows an update button in the server list sidebar if the current version is from the main branch and a newer commit is available.

(Note: the button will also show up if the current commit is from another branch, but also exists on the main branch)

Fixes #148

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation comments
• [x] My changes generate no new warnings