Keyboard shortcuts for navigation

[nbachiyski]

Current: you need to double-click to enter edit mode of a translation.

Expected: a lot of the users come from desktop editors, where keyboard navigation is a common feature and speeds up translation a lot.

See #52 about pressing Esc to close the editor (an easier to implement shortcut.)

• PageUp? and PageDown? to move between rows - will automatically move between pages as well
• Esc to close the editor
• Shift Enter to save the current translation - moves to the next plural form if there is one, automatically moves to the next row/page otherwise.