Add icons for self-hosted identity providers

[FibreTTP]

Adds icons for:

• Authentik (source)
• Authelia (source)
• Keycloak (source)

They are all very popular in the self-hosted space. All optimised with Pingo.

[Kvasenok]

Hi @FibreTTP A few suggestions, if you don't mind. The Authelia icon has some unnecessary margins. Here is an icon remade from SVG without margins. The file size is bigger though. The source is here.

Also, if you decide it needs a dark mode variant due to dark gradient, here is one.

[FibreTTP]

Thanks for that, @Kvasenok. I've optimised the logo you sent and replaced the one with the margins.

If users had an explicit choice between logo variations, I would include the white logo - but given that there are other icons in the repo that are currently as dark as the gradient Authelia logo, and that it's still fairly readable, and that I think it's easier to identify with the colour vs the shape, I'm not going to add it in. Obviously I'm happy to include it if it's blocking this merge.

[Kvasenok]

@FibreTTP No, it was just a suggestion. I don't think a white icon can affect the merge. The services popularity might.

[jamie-mh]

Thanks!