Create `pages` and `page_templates` tables
Implement Pages admin page
Add navigation links to Pages page
Make Smarty search template in database first

If not, fallback to file template
Implement Pages editor from Adminpanel

Store the content for different pages in database, and let admin to
change them. If there isn't content for page in database, use file
template.

You can edit page for specific template (mpos, mmcFE, mobile), or edit
one page for all templates (common template). There is priority for
templates:
- if template-specific page is active - smarty will use it
- else - it will try to use common page, if it is active
- else - smarty will use origin file template

This is basic commit, where only two pages are supported:
'gettingstarted' and 'support'. Developer can easily add new pages later
(just add record to DB)
Fix length validator for $username field