Add support for Bitbucket Server OAuth2 (#64179)

Docs here: sourcegraph/docs#561

This PR adds support for using Bitbucket Server OAuth2 application links
for sign-in and permission syncing.

When used for permission syncing, the user's oauth token is used to
fetch user permissions (and now permissions are fetched via the server).

## Test plan

Tests added and updated.

## Changelog

- Sourcegraph now supports Bitbucket Server OAuth2 application links for
user sign-in and permission syncing.

Author: pjlast

client/web/src/auth/SignInPage.tsx

@@ -222,6 +222,9 @@ const ProviderIcon: React.FunctionComponent<{ serviceType: AuthProvider['service
         case 'bitbucketCloud': {
             return <Icon aria-hidden={true} svgPath={mdiBitbucket} />
         }
+        case 'bitbucketServer': {
+            return <Icon aria-hidden={true} svgPath={mdiBitbucket} />
+        }
         case 'azuredevops': {
             return <Icon aria-hidden={true} svgPath={mdiMicrosoftAzureDevops} />
         }

client/web/src/auth/SignUpForm.tsx

@@ -226,6 +226,8 @@ export const SignUpForm: React.FunctionComponent<React.PropsWithChildren<SignUpF
                                         <Icon aria-hidden={true} svgPath={mdiGitlab} />
                                     ) : provider.serviceType === 'bitbucketCloud' ? (
                                         <Icon aria-hidden={true} svPath={mdiBitbucket} />
+                                    ) : provider.serviceType === 'bitbucketServer' ? (
+                                        <Icon aria-hidden={true} svgPath={mdiBitbucket} />
                                     ) : null}{' '}
                                     Continue with {provider.displayName}
                                 </Button>