Resolve "Database templates"

Closes #153 (closed)

TODO

• Make it possible to import and export the file field or split that out to another issue (https://gitlab.com/bramw/baserow/-/issues/408).
• Add an endpoint to install a template.
• Expose the default selected endpoint.
• Allow the installation of a template via the web-frontend.
• Add the sync_templates command to the install on Ubuntu docs, Cloudron, demo environment, and check where else it has to be added.
• Add documentation about how to create a template, and how to add it to the code.
• Add a couple of templates.
• Fix bug where the filter value pointing to a select option is not exported and imported correctly.
• Left bottom corner of the templates modal doesn't have a round corner.
• Import out said two applications were imported, but it was only one.