Display cost per square meter

See merge request !8

Rework a bit on top of @bnjbvr's MR to add basic email notifications
support.

* Cleaned the code a bit.
* Add a `Date` and a `Message-ID` header to improve the spam score.

Thanks a lot @bnjbvr's for the MR!

Optimize photo comparisons

Closes #41 and #59

See merge request !9

Fix image slider perspective

See merge request !3

* Remove the text-decoration due to the use of links
* Make the link element occupy the whole table cell, without padding
* Use `router-link` instead of `a` tag

Add support for Foncia backend from Weboob

See merge request !5

Thanks @bnjbvr for the contribution!

This closes #15.

Some Weboob backends were returning multiple phone numbers
(comma-separated) or phone numbers in different formats (with or without
spaces, with or without separating dots, etc).

This commit harmonizes all of this so that all phone links will be
rendered correctly and be usable to call in a single click.

Related, https://git.weboob.org/weboob/devel/merge_requests/42.

Weboob does handle the raised exceptions when fetching housing posts
through the `WebNip` class. However, when fetching details, we directly
call the Weboob backend method and no exceptions handling is done.

When importing data with a `crontask`, websites may be offline for some
time, especially at night, then leading to a failure in import process.

Let's just handle such exceptions so that an offline website does not
break the whole import process. Details from the temporary offline
websites which could not be fetched will be fetched at next run.

This closes #60.

More informative error message when matching accidentally a wrong postal code. See #64 for more context.

**Note**: This new feature requires a modification of the SQLite database used by Flatisfy. If you don't care about your current database, you can just remove it and do a reimport. Otherwise, you should do the following modifications manually:

* Add a [flatisfy_constraint](https://git.phyks.me/Phyks/flatisfy/blob/ed7e9dfc1a9090a0a06b3efc4b33ba58b4a13299/flatisfy/models/flat.py#L89) VARCHAR field in the flats table.
* Create [this table](https://git.phyks.me/Phyks/flatisfy/blob/ed7e9dfc1a9090a0a06b3efc4b33ba58b4a13299/flatisfy/models/public_transport.py#L24-32) for public transport data and [this one](https://git.phyks.me/Phyks/flatisfy/blob/ed7e9dfc1a9090a0a06b3efc4b33ba58b4a13299/flatisfy/models/postal_code.py#L24-34) for postal codes data.

Closes #58

See merge request !10

Tcl stations GPS coordinates are given as `lng, lat` and not `lat, lng`
hence it was not working because the found stations were too far from
the housing post location, as reported by @bnjbvr.

This commit should fix this.

There was a bug in `data.py` which made it load the same data as many
times as the number of postal codes in constraints. This is now fixed.

All the backend part has been rewritten to handle multiple constraints
in the config (== multiple queries).

Also did some linting.

Still to be done: frontend part and doc.

This closes #58.