Redesign export dialogs, improving form validation.

I've merged the two export dialogs into one with a dropdown at the top to select between exporting student scores and a full export. Mostly to make sharing code between export types easier.

The form now validates the mapping file before you click export. Participants missing from the mapping file are shown inside the dialog instead of creating one notification per participant. (See #254 and #247)

added 11 commits

• c164c3ed...76f659ad - 6 commits from branch master
• 7df62a69 - unify export dialogs and rewrite reverse mapping logic
• 7c6cb209 - Added more validation for mapping files and checking for incomplete maps
• fe9fa53f - Factored out mapping functions and fixed checking if map is complete.
• f868e685 - Use v-model for opening export dialog
• 5f9ea4e0 - Added list of missing students to alert.

Compare with previous version

added 6 commits

• 6da131da - 1 commit from branch master
• befd5100 - unify export dialogs and rewrite reverse mapping logic
• fb366ce0 - Added more validation for mapping files and checking for incomplete maps
• 80505dc8 - Factored out mapping functions and fixed checking if map is complete.
• 1a7a5c66 - Use v-model for opening export dialog
• c0df189b - Added list of missing students to alert.

Compare with previous version