Improve handling of the zoom URL parameter

This PR makes a couple of improvements of the current implementation of the `zoom` URL parameter:
* Make it possible to use `zoom` without having to also specify `page`.
* Handle the user entering a non-existent or misspelled value for the zoom level.
* Tweak the scale value displayed by the `viewBookmark` button to prevent the current rounding issues for certain scale values, e.g. 110%.
* Fix the coding style (add strict equalities and braces for single line statements) in the functions that this PR touches.