Makefile: mv ~/src/{,wesnoth/}wesnoth-tools

Update the Makefile (which is, yes, tailored to my own setup) to
reflect that I had moved my `~/src/wesnoth-tools` directory, where I
keep a copy of `wesnoth_addon_manager`, into a `~/src/wesnoth`
directory.
Makefile: Allow for ignoring old server failures

In the `Makefile`, allow specifying a Boolean (`0` or `1`)
`ignore_failure_to_upload_to_old_addons_servers` parameter (default
`0`) as `1` to cause the script for the `release` target to ignore
failures to release to old add-ons servers (those that do not serve a
current BfW stable release or its corresponding development release,
if any).

At present, the old add-ons servers listed in the Makefile are those
for BfW versions 1.10.x and 1.11.x.
Makefile: Also release to 1.12.x add-ons server

In the `Makefile`, update the `release` target to additionally release
to the new 1.12.x add-ons server.

Releasing to the `1.12.x` add-ons server requires an updated version
of `wesnoth_addon_manager`.