Improve keyboard accessibility by recognizing Tab in the Mint(s) section [Risolto]

Currently several clicks are needed to handle the Mint(s) field. It would be nice to use the keyboard.

There are several ways the Tab character could be handled better in the Mint(s) section:

1. Tab from the Mintmark field to the Mint field could pop up the input box for the Mint field like a click on the Mint field.
2. When a mint from the list is highlighted, Tab could select the mint and move to the Picture field.
3. When a mint has been selected in the Mint field, Tab should move to the Picture field. Currently it returns to the top of the page. 
4. Tab from the Mint field to the Picture field (currently allowed) could open the list of images as a click currently does.
5. When a Picture is highlighted, Tan could select the picture (and move somewhere, probably the Comments field).

Hello,

I fixed some bugs impacting lost focus when using the dropdown lists for mints and mintmark pictures. The new behaviour is:

1. No change. You need to press Enter to open the dropdown list, like a regular (native) dropdown list.
2. You can now press Tab to select the highlighted mint. The focus stays on the dropdown list though (like for native dropdown list), it doesn't move to the Picture field. (Note: I added this also for all other similar dropdown lists throughout Numista). 
3. I fixed the issue which led you to the top of the page. Now you can either press Enter or Tab to select the mint, then press Tab to go to the Picture field.
4. No change (see explanation in 1.)
5. You can now select with Tab, but it doesn't move to the next field (see explanation in 2.)

Note that it could be nice to be able to select the mintmark picture just with the down arrow, without having to open the dropdown list, like for native dropdown lists.
This is not supported yet by the dropdown lists I'm using. See https://github.com/select2/select2/issues/3472.