Bug 32103: Remove flex from tab-content list elements

Bug 32103: Remove flex from tab-content list elements

To test:
1. Go to HTML customization, News, or Pages and make a new entry using the text editor ( CodeMirror )
2. It is hard to focus on the Content field because the wrapper width is very small.
3. Apply patch and try again
4. The CodeMirror wrapper width should be normal again.
5. Check the other language tabs.
6. Also test the WYSIWYG editor, it should also be normal.

Note: I added some CSS to make the labels appear bold.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>