Add further trans, blocktrans tags and ugettext_lazy calls

[ugoertz]

This handles a few more strings that were untranslated.

I am not really familiar with the vue.js interaction, so in particular line 98 in numbas_lti/templates/numbas_lti/management/resource_remark.html should be double checked (but it works in my instance of the LTI provider, so I suppose it is OK).

There are a couple of things that I left out for the moment:

• I did not touch numbas_lti/templates/numbas_lti/management/admin/stress/view.html, since, if I understand correctly, there also is Javascript interacting with the terms (e.g. the pluralization) and I did not have time to figure out how the translation should be handled at this point.
• There are also some untranslated terms (and non-localized dates) in numbas_lti/static/resource-stats.js.
• The admin interface (i.e., translating the names of models).
• sr-only texts in templates (not sure what the best practice here is, but I suppose they actually should be translated?).
• The display of attempt data (numbas_lti/static/attempt_timeline.js).

The admin and sr-only terms I could do. For the javascript bits, I would probably need some pointer to how this should be done in order to get going.

In any case, the main point (which is done already, also without this PR - thanks again for that!) from my point of view is that the interface as it is seen by the students can be translated. Therefore this PR and the additional points I mentioned have fairly low priority.

[christianp]

Thank you very much for this! I've worked out how to translate the strings produced in JavaScript. It requires a separate .po file, which poeditor doesn't seem to accommodate, so I might have to set up another project there in order to translate those.

Text under the class sr-only is read out by screenreaders, so should be translated.

I've added translations for the model names in the admin interface.