Improve display of the three pages of the plugin.
- label dashboard: instead of a single page with 5 sections, create a tabbed display with one tab per section. Similar to pluginManager section
- label dashboard: display label's description
- labels & computer page: remove the "Exclusive jobs" section which is redundant with Linked Jobs section, and instead display a warning in the Linked Jobs section for each label that can run on only one node
- in all pages, use a two-column display like in the Configure System page to make page more readable
- less links on the page?