[5.x] Ability to customize columns in the Asset Browser

[duncanmcclean]

This pull request makes it possible to customise the columns in the Asset Browser.

Closes statamic/ideas#202.
Partially addresses statamic/ideas#1002.

To Do

• UX: Figue out the best way to handle folders when the first column isn't the filename
• UX: When switching between Grid & List view, the customize column button disappears/reappears. What's the best way to handle this? (maybe it's fine 🤷‍♂️)
• The setColumns logic is duplicated in BrowserController/SearchedAssetsCollection. Figure out a way to DRY this up.
  • In an ideal world, we'd be using a resource for the "all assets" listing request. However, due to the fact we're returning multiple types of "things" (folders & assets), we're not doing that here, hence the duplication. Need to think about the best place to put this. 🤔
• Ensure it works fine with the Assets fieldtype
• Fix failing tests