Bump bleach from 2.0.0 to 3.1.1 in /requirements

Bumps [bleach](https://github.com/mozilla/bleach) from 2.0.0 to 3.1.1.
- [Release notes](https://github.com/mozilla/bleach/releases)
- [Changelog](https://github.com/mozilla/bleach/blob/master/CHANGES)
- [Commits](mozilla/bleach@v2.0...v3.1.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/common.txt

@@ -18,7 +18,7 @@ SQLAlchemy==1.3.0
 WTForms==2.1
 Werkzeug==0.15.4
 alembic==0.9.5
-bleach==2.0.0
+bleach==3.1.1
 blinker==1.4
 html5lib==1.0b10
 itsdangerous==0.24