login: add /login & /logout routes, rename 'sign in' to 'login'.

* Replace /session/new with /login and /session/sign_out with /logout.
* Rename 'sign in' to 'login'.

This changes are to make urls cleaner and terminology more consistent.

app/javascript/src/styles/base/040_colors.css

@@ -161,7 +161,7 @@
   --bulk-update-request-approved-color: green;
   --bulk-update-request-failed-color: red;
 
-  --sign-in-link-color: #E00;
+  --login-link-color: #E00;
   --footer-border: 1px solid #EEE;
 
   --jquery-ui-widget-content-background: var(--body-background-color);
@@ -434,7 +434,7 @@ body[data-user-theme="dark"] {
   --wiki-page-versions-diff-ins-background: var(--green-0);
 
   --low-post-count-color: var(--red-1);
-  --sign-in-link-color: var(--red-1);
+  --login-link-color: var(--red-1);
   --new-artist-color: var(--red-1);
   --remove-favorite-button: var(--red-1);
   --fetch-source-data-border: 1px solid var(--grey-4);

app/javascript/src/styles/common/page_header.scss

@@ -41,9 +41,9 @@ header#top {
       font-weight: bold;
     }
 
-    li#nav-sign-in a {
+    li#nav-login a {
       font-weight: bold;
-      color: var(--sign-in-link-color);
+      color: var(--login-link-color);
     }
 
     li.forum-updated a {