Change system branding

[mariobehling]

• Change name to "eventyay"
• Use same logo as on eventyay-talk
• Change to "powered by eventyay" with link to eventyay.com
• Change the name "pretix" in all text fields to "the system" or similar. It should make sense.
• In Readme.md change from Pretix to Eventyay-Tickets

Compare to https://github.com/fossasia/eventyay-talk/pull/69/files

[progmatic-99]

working on this

[AviGawande]

due issue #103 being closed and i had to raise the PR to the main repo

[mariobehling]

Yes, please go ahead. Reopened the issue. Please finish the remaining items. Thanks