Add an on/off toggle for "Custom tags" option

[vandermerwed]

Should be able to toggle the Custom Tags option on and off - Since we have roam/css now this feature is maybe less needed.

Consider deprecating this and writing a guide on how to use tags in roam/css. Will need to put up a notice in theme settings for users to copy all tag colours and switch to using tags in Roam. I don't want people to lose their tags.

Adding the toggle is probably a good way to do it because I can turn tags off by default and users will wonder where their custom tags went. This will make them check out the settings.