Disable (and hide) Back button in new project form

Back button is not implemented, and some backend API changes will need
to be made before it can be implemented. For example, when the user goes
back to the "upload initial data" step, we'll need to call an API to
delete the just-uploaded .zip file and reset the project to empty (zero
entries) again.
Shorten status message for duplicate project code

Error message for "project code already exists" was overflowing onto two
lines, which looked ugly. Shorten it so it doesn't make the dialog below
it move around any longer.
A few improvements for RTL interface

Add some spacing on the right, as well as the left, of some button text,
so that Font Awesome icons will be placed with good spacing no matter
whether they have ended up on the left or the right of the button text.
Also make sure the progress bar for uploads is pulled in the correct
direction, left or right, so that it stays under the "Drop files here"
box. (It was previously pulled left no matter the interface direction.)