common: Implement basic templating for cockpit resources

These are very similar to autoconf configure templates and consist of
@@variable@@ to substitute 'variable' for a value. There is an API
that implements this, and we now use it for index.html

This will be used for substituting package checksums for package
names in resource files.
ws: Use templates to expand paths in resources

Strings like @@shell@@ are expanded to the full checksum and/or
name of the package in question, including addition of the relevant
host to load from. This allows package to refer to files from other
packages simply.

The shell package needs to be expilictly qualify all of its paths
since it is not loaded into its own frame/document but replaces
the login page.

Resource URLs now all have the same prefix, and automatically
detect whether a checksum or package@host style is in use.

They look like this:

/cockpit/xxxxxchecksumyyyyy/path/to/resource.file
/cockpit/package/path/to/resource.file
/cockpit/package@host/path/to/resource.file

The prefix '/cockpit' was chosen so that when other web apps
do deep embedding of our components, the prefix should not
conflict.
agent: Resolve checksums in the agent instead of web service

This will make life easier for people embedding Cockpit if they
end up handling the resource channels and resource loading
themselves.

Closes #1316
lib: Include both minified and non-minified jQuery and bootstrap

The latter are useful in debugging
agent: Add "accept": "minified" resource channel option

If set, this will choose a minified version of the file if available.
Minified files sit next to the normal files and have an suffix of '.min'
just before their real file extension. For example:

  jquery.js
  jquery.min.js

This is in a similar vein to the content selection that the HTTP
Accept and Accept-Language headers provide.
tools: Rework how the js-lint and minify tools work

js-lint now cats the files together, and provides the input
to minify.

Install different minified and non-minified file names, for use
during debugging and non-debugging, and making use of the content
selection of minified files in the previous commit.
shell: This module becomes more normal

Rename cockpit.js to shell.js, since cockpit.js will be API in a
base package.

Also there's no need to list everything in the manifest. Loading
of resources happens via well known paths. The shell has such well
known paths. Loading of javascript in other packages will happen via
<script> tags. Do something similar here in the shell.

Note that the use of a lot of split javascript files that get
combined is pretty shell specific. Eventually we'll usually have one
javascript file per iframe/component to be loaded.

Closes #1319