Update csv email

Can be done after or with the django update. The CSV download email can use an update. Suggestions:

- Use the SITE_NAME from settings rather than hard-coding 'I-analyzer'
- ~~Update the organisation name (currently Digital Humanities Lab) - also make this configurable in settings?~~
- ~~Update the logo link in settings (or just update the image on dhstatic)~~
- Add a link to the downloads page.
- The downloads page currently offers options that the direct download link doesn't, like excel-friendly encoding. Two possible solutions:
- Change the email so it presents the link to the download history as the primary option, encouraging users to download through there.
- The direct download link in the email should not lead to the API, but to a frontend page that offers download options.
- The plain text version is messy and does not include the actual link. This is an issue for unit testing, as well as accessibility.
- I would not be opposed to simplifying the HTML for a more readable template and more plain-looking email.