unminify: Fix source map extraction for hash-named files.

Apparently, our old unminify logic relied on the fact that the
filenames displayed in tracebacks were of the form "app.js" (and the
`app.js` copy of the source map in the appropriate
`/home/zulip/deployments/`).  The correct behavior is to just look up
the source map for the appropriate hash-named
`app.a40806b10565c1dee5bf.js` type file.

We fix this with a few small tweaks to the regular expressions.  I
wish this file had reasonable unit tests.

zerver/lib/unminify.py

@@ -30,10 +30,10 @@ class SourceMap:
         out = ''  # type: str
         for ln in stacktrace.splitlines():
             out += ln + '\n'
-            match = re.search(r'/static/(?:webpack-bundles|min)/(.+)(\.[\.0-9a-f]+)\.js:(\d+):(\d+)', ln)
+            match = re.search(r'/static/(?:webpack-bundles|min)/(.+)(\.[\.0-9a-f]+\.js):(\d+):(\d+)', ln)
             if match:
                 # Get the appropriate source map for the minified file.
-                minified_src = match.groups()[0] + '.js'
+                minified_src = match.groups()[0] + match.groups()[1]
                 index = self._index_for(minified_src)
 
                 gen_line, gen_col = list(map(int, match.groups()[2:4]))