Add optional metadata form in ERC creation

[Fmazin]

This PR should resolve #146.

With this pull request a fourth form page for the ERC creation is added. On this form page you can edit the DOI, the languages of the paper(ISO 639-1 codes) and the keywords.
Full optional metadata page:

They are also shown on the metadata overview on the ERC site:

Checks

I have included a few checks for the metadata.

1. The DOI is checked for validity:
   

2. The keywords should always be at least two characters long.
   

3. 1. The codes must be from the ISO Standard.
      
4. 2. The languages must be unique.
      

If any of the checks are to much or a more general problem arises please let me know.

[njakuschona]

The code looks good so far. I would suggest to test if the reset and tab change works as expected after the merge. I can take care of it after the merge.

[nuest]

Looks good to me (judging from the screenshots), I trust @NJaku01 to confirm it works :-)

[nuest]

@NJaku01 please go ahead with the merge, ideally you can follow up with the mentioned tests right afterwards.

[njakuschona]

@nuest I tested the reset. I fixed some minor errors; the fixes are in #204. After @Fmazin reviewed these changes, I would merge them and merge the tests afterwards.