Add mix task to check website availability and cleanup list of companies

[tonnenpinguin]

Hi!

I took the liberty of introducing a little mix task that allows you to quickly check which websites are not available at the moment.

I also went through all the flagged websites and checked manually if the company still shows some signs of life.

Here's the list of companies I removed in the process:

approva: inactive on github since 2019 canvas: inactive on github since 2019 askcharlie: website down, no github aviacommerce: inactive on github badger: website down, no github chinch_financial: inactive since 2020 codecorps: inactive since 2019 slickspaces: rebranded to operto daybit: inactive since 2021 diatom: inactive since 2019 exeq: deleted github fred: no github gitmonitor: no github grok-interactive: inactive since 2018 hiringsolved: no github icicletech: inactive since 2019 ispirata: inactive since 2021 kmonline: inactive since 2021 neon_tree_solutions: no github palar_support: no github patternmatch: inactive since 2021 qixxit: no github (possibly bought by lastminute.de) rexpad: no github salam: no github the_club: no github voicelayer: no github wunder: inactive since 2021 xerpa: inactive since 2021 youprop: no github