Simplify Logo requirement from users. (#373)

* Simplify Logo requirement from users.
- Only require 1 logo file, instead of a `small` and `large` one.  Just require `logo`.
- Update frontend sso that primary header logo will ALWAYS be owncast logo.
- User's logo will remain in "user content" area.

* Commit updated API documentation

Co-authored-by: Owncast <owncast@owncast.online>

webroot/styles/app.css

@@ -39,6 +39,7 @@ button[disabled] {
   pointer-events: none;
 }
 
+/* accessibility things */
 .visually-hidden {
   position: absolute !important;
   height: 1px;
@@ -58,6 +59,8 @@ header {
 
 #logo-container {
   background-size: 1.35em;
+  padding: 1.15rem;
+  background-image: url(/img/logo.svg);
 }
 
 #chat-toggle {

webroot/styles/user-content.css

@@ -3,7 +3,8 @@
   width: var(--user-image-width);
   height: var(--user-image-width);
   max-height: var(--user-image-width);
-  background-size: calc(var(--user-image-width));
+  /* background-size: calc(var(--user-image-width) - 2em); */
+  background-size: cover;
 }
 
 .user-social-item .platform-icon {