Restructure /accounts/ URLs

See https://salsa.debian.org/freexian-team/debusine/-/blob/devel/docs/reference/devel-blueprints/url-redesign.rst?ref_type=heads

accounts contains a mix of authentication related endpoints, which can be restructured (see also #553 (closed)):

• /<scope>/accounts/accounts/bind_identity/<name>/: move to /-/signon/bind_identity/<name>/ (!1429 (merged))
• /<scope>/accounts/accounts/oidc_callback/<name>/: move to /-/signon/oidc_callback/<name>/ (!1429 (merged))
• /<scope>/accounts/login/: move to /-/login (!1421 (merged))
• /<scope>/accounts/logout/: move to /-/user/logout (!1422 (merged))

Moving the bind_identity/oidc_callback views requires an update of the Applications configuration in Salsa once the change is deployed