Replace lock icons

[toby63]

Suggested by @streaps here: #17 (comment)

It's about this icons:
https://github.com/mumble-voip/mumble-theme/blob/master/lock_locked.svg
https://github.com/mumble-voip/mumble-theme/blob/master/lock_unlocked.svg

Related to this pull request: mumble-voip/mumble#3929

They look inconsistent to the general theme, so maybe someone can improve them.

[toby63]

My idea:

• locked state: imo it should be filled with colour, also no keyhole is necessary.
• unlocked state: It should look more like this: https://s3.envato.com/files/235925363/Access%20Granted%20590x332.jpg
  🤔 But overall maybe the open lock is not visible enough, though the green colour might be enough...
  An alternative would be a key symbol or something similar.