Rename caching.py -> client_caching.py.

The word caching can be ambiguous, and what's worse is this file
previously contained a decorator for server-side caching of a
response and a class for client-side caching of files. It was
confusing. This new separation and naming should make it easier
to find what you're looking for.

frontends/etiquette_flask/backend/client_caching.py

@@ -21,6 +21,8 @@ class FileEtagManager:
 
     We use the file's MD5 hash as the ETag, and will only recalculate it if the
     file's mtime has changed since the last request.
+
+    Note, this class does not store any of the file's content data.
     '''
     def __init__(self, maxlen, max_age, max_filesize):
         '''

frontends/etiquette_flask/backend/common.py

@@ -13,7 +13,7 @@ from voussoirkit import pathclass
 
 import etiquette
 
-from . import caching
+from . import client_caching
 from . import decorators
 from . import jinja_filters
 from . import jsonify
@@ -45,7 +45,7 @@ site.debug = True
 site.localhost_only = False
 
 session_manager = sessions.SessionManager(maxlen=10000)
-file_etag_manager = caching.FileEtagManager(
+file_etag_manager = client_caching.FileEtagManager(
     maxlen=10000,
     max_filesize=5 * bytestring.MIBIBYTE,
     max_age=BROWSER_CACHE_DURATION,