Create revision history entries for secret-changes via API

[JensH0rnung]

Feature:

Changes to a secret via the API now create an entry in the revision history of this secret. Before, entries were only created if changes were made in the browser.

Styling the revision history:

• Separate icons for the change-type in their own column
• Show both icons, if changes include secret_data AND metadata
• Add fixed table-layout with percentage column widths
• Truncate long usernames with tooltip showing the full name

[nkoch-seibert]

I think this might be an issue for file secrets, peek_data returns raw bytes, but the payload parameter needs to be json-serializeable.

In the view, we guard it like this:

if secret.content_type == ContentType.FILE and isinstance(current_data, (bytes, bytearray)):
      current_data = {'file_content': base64.b64encode(current_data).decode()}

[JensH0rnung]

Added the guard:
cdc4ed3

[nkoch-seibert]

Nice! Though I still think there's an issue here, that I missed on my first review.
peek_data only returns the file contents; if the payload was stored with a file_name, this would erase that on metadata changes. I think the best way around this would be to make the payload optional in the RevisionService.save_payload(), and use secret.current_revision if it's None there.
That way, we don't need all this peek_data and the file-type guard, it just collapses to

elif instance.current_revision:
    RevisionService.save_payload(secret=instance, actor=self.request.user, payload=None)

We can then also do the same for the browser view.