Improve user details page and add new study page with search functions.

The same link of a search hit can point to different pages. A page can
also be needed from multiple places with most the same content but a some
parts may be different (e.g. a different button to denote different
actions on a user details page).
Add user teaches and lesson details pages. Streamline the history and use
history.back as much as possible. Support swipe right to go back to
previous page on most pages.

The current model can lead to infinite depth of navigation. It may be
needed to have a quick way to go back to the home page.