Unit: forbid overwriting source texts from the web interface

Overwriting source texts in this context is undesired, which ended up in
mangled DB content because of #278.

The reasoning behind allowing to edit source texts can be explained with
the fact that terminology units have been sharing the editing/validating
form with regular units. It's not the case anymore — these are very
different contexts with different use-cases.

Fixes #277.
Cleanup: aligned function parameters
Cleanup: inverted conditional branch for improved readability
Settings: added missing key-value pair
Editor: don't mess up markup when injecting it to the page

Turns out a jQuery bug [1] corrupts HTML attribute values that look like
self-closing tags and in some cases this was altering the value of the
hidden input fields.

Besides, too much jQuery was being used to incorporate the final markup to
the page, and this commit simplifies the approach.

Fixes #278.

[1] http://bugs.jquery.com/ticket/13821
Cleanup: use relative imports, adjust to line length
Cleanup: removed dead code
Assets: don't duplicate comment
Webpack: split vendor code from generated bundles

Avoids repeating the same library code in app-specific bundles.
Webpack: extended comment
npm: updated dependencies
Makefile: added CWD
Assets: cd to dir for building
Assets: ensure dependencies are up-to-date when building
Use a platform-independent (read: Windows-friendly) way to format dates with no leading zeros
Under Windows, skip setting locale at all

This should prevent nasty "UnicodeDecodeError: 'ascii' codec can't decode..." errors when formatting dates
Indicate obsolete units in the report
Get rid of initial XHR on Overview pages (Fixes #291)
PEP8-related cleanup
Assets: reverse `js_vendor` and `js_common`

In `js_vendor` we don't actually include libraries but shim them so we're
able to use them as modules, therefore this bundle needs to come after
`js_common`. This dependency/restriction will go away once we switch all
our client-side code to use webpack and CommonJS modules.