Add EntityField for ER diagrams

Added the EntityRelationship diagram type and added EntityField to go with it. Right now you can edit the name, type, and keys associated with a field in an entity.

I was planning on making the fields columnized, but I was running into issues getting the column widths to stay persistent for each field, so right now each field has its values formatted for one cell.

Known issue: Only the field's name is saved when exported; the type and keys don't save. Fill fix this next sprint.