CSS: remove hardcoded style from templates

Fixes shaarli/Shaarli#29

Style elements refactored as follows:
- use existing ids and classes if possible,
- else, define new ones and stick with the existing naming convention,
- remove hardcoded style attributes from RainTPL templates.

Exception:
In tpl/tagcloud.html, the display size of each tag is computed at page
generation.

TODO: tpl/picwall2.html does not seem to be used...

Signed-off-by: VirtualTam <virtualtam@flibidi.org>