Add support for file uploads

See merge request !17

And change the default retry to 1 second instead of 0.

Aside from the Client builder, mwapi is simple functional interfaces.
UploadRequest is a better fit for mwbot. Shift it over and introduce a
very stubbed `File` type for it to live in.

The main difference from normal requests is that uploads require sending
files in multipart requests. These requests are also generally not
retriable since it might be referring to a Stream (not yet implemented)
that can't be cloned. The logic of making requests has been extracted so
it can be called independently.

As part of this refactor, we had to move the retry-after header handling
into the error itself. Unfortunately this is a breaking change since it
changes the error types, so we can make a few other changes like
replacing `should_retry()` with `retry_after()`.

By default, files are uploaded in chunks of 5MB. This can be adjusted
depending on the wiki you're uploading to and bandwidth. If the wiki
supports it, async mode will be used too. Upload-by-url is not supported
yet, mostly because the MediaWiki implementation is pretty busted and
you're better off downloading it on your own.

Bug: T319098

Fix clippy warnings

See merge request !19

The default is already info, so all these overrides do is
prevent people from overriding them via the environment variable.

Extract Client::token() with the get-or-fetch logic for CSRF tokens

See merge request !16

This will be used later on by the uploading code that can't use
post_with_token().

Clean up etag handling

See merge request !15

Make it pub(crate) and just access the field directly instead of
going through a getter and setter. Those two functions are now
deprecated and are expected to be removed in the next breaking release
if there are no objections.

Have Wikicode store a title instead of trying to parse it

See merge request !14

Because of T324431, HTML is somtimes ending up in the <title> element,
so it isn't usable as an actual title. In most cases, we know the title
since we requested it from the server, so just store that title since
we at least know it's the correct title.

Most importantly this makes transform_to_wikitext() work for pages with
customized titles.

Bug: T326490

refactor tests

See merge request !12

Remove mwseaql, split into separate repository

See merge request !11

Now at <https://gitlab.wikimedia.org/repos/mwbot-rs/mwseaql/>.

allpages generator

See merge request !8