Implement auto-completion using At.js

Here's how it works:
- in every bbcode-enabled textbox the autocompletion is silently enabled.
- press the BBCode character ([) to display the autocompletion menu.
- press enter or tab to insert the autocompletion hint.
- press tab to go from one parameter to another ([url=%cursor][/url] after a TAB becomes [url=whateveryouhavewritten]%cursor[/url]).
- the library is automatically loaded from the jsDelivr CDN, and only when the user enables the feature.
- every bbcode is supported except the [list] tag, which is too complex to implement.