Add Skip Link - #1774

In Kanban sometimes my focus is on the first column's edit button by default instead of before the skip link

In Rich Text pressing enter when the skip link is in focus doesn't produce any result

A description of some behaviours I found which may or may not be desired (Firefox desktop):

• In Kanban, skip link skips tag button + tag/drag toggles as well as generic main toolbar
• In Pad + Presentation, can’t escape editor with ESC in order to tab through interface - have to click outside of the editor manually; editor toolbars skipped
• In diagram - focus gets ‘lost’ after navigating through right hand side panel; skip link doesn’t appear unless interface manually clicked first outside of the editor
• In document - OO embedded file menu/toolbar inaccessible to keyboard
• For multiple apps, the ‘do you want to store’ modal is the first thing that is accessed through tabbing, not the skip link

Many thanks @zuzanna-maria for catching these. I think the majority are known issues which could not be resolved for this v1 of the skip link. Pinging @dariiing anyway as the last one about storing dialog may not be known.