cover: Handle covers with specific heights:

- Have an explicit contextual class on `cover-container`.
- Use more flexible, consistent and searchable variable name for passing classes to covers.
- Consistently use `'…'` with django variables.
- Give the option to not hide covers to screen readers.
- consitently give a title to the cover container if `alt_text` exists.
- [lists] Remove `.content` which is applying too extensive default styles.

bookwyrm/static/css/bookwyrm.css

@@ -124,6 +124,14 @@ body {
     position: relative;
 }
 
+.cover-container.is-small {
+    height: 100px;
+}
+
+.cover-container.is-medium {
+    height: 150px;
+}
+
 /*
 .cover-container.is-large {
     height: max-content;
@@ -135,14 +143,6 @@ body {
     height: auto;
 }
 
-.cover-container.is-medium {
-    height: 150px;
-}
-
-.cover-container.is-small {
-    height: 100px;
-}
-
 @media only screen and (max-width: 768px) {
     .cover-container {
         height: 200px;
@@ -163,6 +163,11 @@ body {
     image-rendering: optimizeQuality;
 }
 
+[class~="has-height"] .book-cover {
+    width: auto;
+    height: 100%;
+}
+
 .no-cover .cover_caption {
     position: absolute;
     padding: 1em;