Change folder delimiter to semicolons and disallow semicolons from names

[camdendotlol]

The document folder feature added in #433 uses the acts-as-taggable-on gem, specifically this version: https://www.rubydoc.info/gems/acts-as-taggable-on/4.0.0

acts-as-taggable-on stores the list folder names as a single string with a comma delimiting each name. This means that, currently, if you try to add a folder named Blake, William, it will end up applying two folders, Blake and William to the selected documents.

I discussed this issue with Dino and we agreed on the following solution:

1. Change the delimiter to ;, which according to the docs is as simple as calling a delimiter method in the acts-as-taggable-on inititializer.
2. Disallow the semicolon from folder names and display an error if the user tries to add one.

[jamiefolsom]

We can't change the delimiter application-wide; first step here is to figure out if we can change it for just the folder, and not other tag contexts. @camdendotlol @sarastanton