Add channel:X filter

[omarroth]

Let's say I'm on a channel page (/channel/X), the search box should be autopopulated with a channel filter such as channel:X.

Btw, great project! Invidious loads way faster than the official YouTube site on Firefox.

Originally posted by @r-a-y in #126 (comment)

[omarroth]

Added with f7ca81c. Users can search channels with channel:X and user:X, where X can be either a channel name, e.g. BlenderFoundation, or a ucid, e.g. UCSMOQeBJ2RAnuFungnQOxLg. channel and user are interchangeable.

Unfortunately, other filters will not work for channel search, and the official YouTube website doesn't appear to support them either. However, supporting other filters may be possible, it would just take some more experimenting.

f7ca81c also autofills the search bar with the channel name, or the ucid if the channel name contains spaces. I'd like to fix this in the future so filters can be e.g. channel:"Entertainment - Topic".

The last thing to note is that channel search does not work on auto-generated channels, for example Entertainment - Topic or Music, so Invidious does not autofill the search bar.