[Feature, API, UI] Channel specific buttons panel

Not sure if it was already discussed but it would be great addition if we had the customizable buttons panel for channels (or server at least). It can be great QoL change but also can pose several difficulties which I will try to address below.

The goal is to have a user-defined set of buttons which can be added in channel/server settings. They can be set to simply follow specified URL or used by API to fire callbacks on click which can be processed by bots. Basicaly they will have two modes simple url and API call, in the former you set URL and the callback name in the latter.

This will be a great benfit for bot devs and streamers. They will have an ability to bind frequently used commands to buttons for easier access. Ordinary group members will also benefit from fast and clear access to commonly used URLs.

You can also make them Discord roles dependant so only selected roles can view exact buttons.

Limitations:

- Each button must have customizable reactivation time delay with predefined minimum. Might be callback reactivation delay to exclude excessive spam and abuse.

- Warning window should be displayed on click with the notification message of following URL or sending data for user to confirm. This probably can be bypassed by checking "do not show anymore" checkbox.

- Each channel/server must have a hard limit of 10 (?) buttons.

- Adding these buttons in each channel will definetly make Discord DB to go on fire but still I think it will be nice to have them. At least on the Group level so callback will have name and channel ID for bot to work with. Much less data to store and workload increase for D in this case.

Let me know what do you think and if there're problems I can't see.