Improve some styles and UX problems

1. Core tabs are usually hidden on small screens. To avoid that Sliding tabs should be implemented.
2. -----MOBILE-2279----- and other add buttons to FAB.
3. Improve course section selector: The course section selector is hard to see/understand.
4. We should add typeGroups in tsconfig: https://moodle.org/mod/forum/discuss.php?d=370571#p1497480
5. Main Menu: Notifications tab isn't shown in 3.1 sites (site home is).
6. RTE issues:
   1. Bullets/nums not seen.
   2. Resize not working right in Android (when I close the keyboard the RTE keeps being small, when the keyboard is open I can't see the RTE buttons). I tested it on wiki.
   3. Brush icon should be improved.
   4. When the RTE is empty, writing on it sometimes place the text outside of <p></p>, so the text styles are different.
7. When going back a strange effect happens.
8. In login we have this @todo: "Display label and select in different lines". Also, the box is quite narrow when using a list of fixed sites.
9. In CoreLocalNotificationsProvider.showNotificationPopover we're just displaying a Toast with the message. This should be improved to display a notification similar to what we did in the previous app.
10. In iOS 11, there was a padding at top in course contents.
11. Credentials view: cannot navigate through fields in iOS using the virtual keyboard.
12. Check scroll effects on iOS.
13. Click to add a new site and enter the proto 3.3 URL. In the credentials screen, part of the box ends up being behind the header bar.

iOS8:

1. The courses in dashboard weren't seen.