Fix/exported paragraph tags #1485

Closes #1485 (closed)

Summary

We're not stripping tags on export.

I went with the pure PHP approach over our XSS->clean() because it was adding in/allowing the <?xml version=“1.0” encoding=“utf-8”?> which messed up posts, and is more geared towards other areas of the site than activity export. Feel free to let me know if we want to funnel everything through that and I can take a look.

Steps

1. Make a post with the text <p>Test</p>.
2. The activity should not read <p>Test</p>, it should read Test.
3. Code and pre should still be allowed if code-highlighting is on.
4. Editing the activity you should just see Test

Regression Scope

Are there any exported tags we want to keep? This could mess up the message export of a type of activity in the worst case.