Replaced Boostrap 2.3.2 js and css files with the Bootstrap 3.0.0 version.

A lot of things don't look right, the views have to be changed to make them compatible with version 3 of Bootstrap.
WIP: adapt views to Bootstrap 3.

Layout looks almost right, but seemingly nav-lists have disappeared from Bootstrap. More work is necessary for the sidebar and the entries list to work as expected.
WIP: adapt views to Bootstrap 3.
Adapted dropdowns to Bootstrap 3.
Adapted the navbar to Bootstrap 3.
Show unread count badges only if the count is greater than zero.

This helps decluttering the user interface, by removing badges with zeroes.

Actually the span for the badge itself is always created, but in bootstrap 3 it is hidden if the span has no textual content.
WIP: adapt alert styles to Bootstrap 3.
Fixed start page grid.
Changed alert type for the "upload OPML" div.
Alerts are displayed fixed to the top of the viewport and have responsive width.
Text in alerts is centered.
Properly style links in alerts (new in Bootstrap 3).
Adapted data import progress bar to Bootstrap 3.
Adapted sidebar to Bootstrap 3.
Removed redundant id attribute and cleaned up another html attribute.
Use data- attributes to open the data import popup, instead of doing it with javascript code.
Bottom margin removed from alerts.

The "margin-bottom: 20px" property in alerts was like an invisible "band" that stopped the user from clicking on buttons even though they were visible below the alert. Less confusing this way.
Open the new folder popup using data- attributes instead of javascript code.
Added "collapse" class to sidebar folders.

It seemed to work fine without that class, but according to the bootstrap docs it should be added to the collapsible element.
Changes in the sidebar to make it work like an accordion.

A bit of mixing between panel-group and list-group markups is used. It seems a bit weird but it seems to work fine. I was happy using list-groups but I haven't been able to make the accordion example in the bootstrap docs work with list-groups, only with a panel-group.