Provider list fallback and list of providers in both servers' /links-endpoints

[CasperWA]

Fixes #450
Fixes #454

We experienced that providers.optimade.org was down for some time, hence this will make sure to use some fallback's to try and retrieve the list of providers.

Another thing is fixed in this PR. Namely that the list of providers was not included for the index meta-database.
There was a good reason why: The same links MongoDB collection was used in-memory when starting both servers within the same environment.
This has been fixed by the use of replace_one/replaceOne thanks to some help from @shyamd.

[CasperWA]

Note: This version lost two commits (where I removed the providers submodule). Hence the get_providers() function does not use data from the submodule, while the providers.json is still accessible through optimade.server.data.providers.

I have left it like this for now, but it should go one way or the other:

1. Either we remove the providers.json and don't use data from the submodule whatsoever, or
2. We implement another fallback, i.e., to use the providers data from the submodule if the URLs fail, just before emitting a warning if this can also not be found ...

Personally, if we're holding on keeping the submodule in, I'd prefer to also then use it.

[codecov]

Codecov Report

Merging #455 into master will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
+ Coverage   91.26%   91.65%   +0.39%     
==========================================
  Files          60       60              
  Lines        2815     2828      +13     
==========================================
+ Hits         2569     2592      +23     
+ Misses        246      236      -10     

Flag	Coverage Δ
#unittests	91.65% <100.00%> (+0.39%)	⬆️

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Impacted Files	Coverage Δ
optimade/server/main.py	95.08% <100.00%> (+0.34%)	⬆️
optimade/server/main_index.py	95.16% <100.00%> (+0.42%)	⬆️
optimade/server/routers/utils.py	98.01% <100.00%> (+6.85%)	⬆️

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[CasperWA]

Note: This version lost two commits (where I removed the providers submodule). Hence the get_providers() function does not use data from the submodule, while the providers.json is still accessible through optimade.server.data.providers.

I have left it like this for now, but it should go one way or the other:

1. Either we remove the providers.json and don't use data from the submodule whatsoever, or
2. We implement another fallback, i.e., to use the providers data from the submodule if the URLs fail, just before emitting a warning if this can also not be found ...

Personally, if we're holding on keeping the submodule in, I'd prefer to also then use it.

I have implemented option 2. from above now, and also added tests to make sure every possibility reachable is tested and returns/warns what is expected.