Add filter-by-language to study list

[rhodricusack]

There are many English studies on lookit and at present only one Japanese one. We are worried that on the studies list page, the mass of English studies will be off-putting and confusing, and rarely useful. So, this commit:

• adds a language field to each study (studies/models.py), giving the default of 'en'
• on the study list page web/templates/web/studies-list.html we then add a new drop-down to filter by language, currently we three options: All, English or Japanese. For English this defaults to All but if the URL country code is ja, it defaults to Japanese (thus filtering for Japanese studies). Note this is stored in a browser cookie and so will only be picked up the first time you hit the page.

[sonarcloud[bot]]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

1 New issue
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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