admin: Use array of filters instead of generator
admin: Update the Tips and Tricks

Also, take code chunks from `tables.html` and `filters_list.html` and
put it in new files for readability reasons.
admin: Change wording in tables.js
admin: Conditionally concatenate filter terms

Concatenate filter terms of model filters that have the "=" sign between
them. Also, add a setting (`ADMIN_FIELD_SIGN`) for the administrator to
change that sign if he/she wants to.
admin-ui: Improve advanced search

* Rename "Basic Search" to "Category View" and "Advanced Search" to
"Text View".
* When a user types an incorrect search term the error is now visible
beneath the input of the filter.
admin-ui: Improve style of popovers
admin-ui: Improve style of tips