Add TagHelper functional test.

- Added an end-to-end test that verifies all content behaviors and functionalities of tag helpers.
- Added some common user scenarios to verify that the system works how we expect.

#1116

**Note:** Wasn't sure to what extent I should hold back on testing these so they may be too extensive.
I'm ok with anyone using Regex, but please don't make us read them. Just make a small comment saying this does this :smile: