Increase md typography font size

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'increase-font-size' into 'master'

Increase font size

Increase markdown typography font size to 15px.

Affected:

* wiki
* issue/mr description
* md files
* comments

Fixes #1662

See merge request !1265