CSS: remove hardcoded style from templates

### PR content
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

### Appearance
Though I've aimed at sticking as close as possible to the original CSS, minor changes were made:
* QRcode image resized from 13x13 (blurry) to 16x16 (the icon's res),
* cleanup in `tpl/install.html`:
  * removal of a non-closed, and useless `<div>` (please correct me if it is of any use)
  * table: some superfluous styling elements were removed: the html code is a (tiny) bit cleaner, and the styling is done in the CSS.

If needed, these modifications can be submitted in a separate PR.

### Todo
* tpl/picwall2.html does not seem to be used, is it necessary to process it? If so, how to use it?

### Hints for reviewers / How to test this PR
I've used the following environment to develop and test this patch:
* a vanilla Shaarli for display reference,
* a development Shaarli,
* a couple desktop browsers (Firefox and Chromium, on Linux) to compare what's displayed.

As looking to each modified template's side-by-side diff is a bit tedious, this PR is best reviewed by:
* applying the patch on a dev environment,
* looking at a modified template file,
* identifying the modified bits:
  * disparition of hardcoded `style=""` attribute,
  * apparition of CSS classes and ids.
* looking for those new elements in `inc/shaarli.css`,
* comparing the displayed pages with a reference Shaarli version.