allow address_layer_filter to be user configurable

[missinglink]

As discussed in https://github.com/pelias/api/pull/1274#issuecomment-651927481 the address_layer_filter isn't everyone's cup-o-tea so we should add some ways of configuring or disabling it on a per-installation basis.

As a quick reminder, I'm talking about the bit of code which produces these messages:

The functionality was introduced in https://github.com/pelias/api/pull/1274 and shouldn't be confused with this similar functionality https://github.com/pelias/api/pull/1219 and this one https://github.com/pelias/api/pull/1215 🤷‍♂️

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I'm opening this initially as a DRAFT PR so we can discuss it and come up with something which'll work for most people.

My initial commit introduces a config flag api.enable_address_layer_filter which simply enables/disables the filter. I defaulted it to true so it's backward compatible.

This would probably be helpful for anyone who's running a small-to-medium sized installation of Pelias, because at under a couple million records you're less likely to suffer from the performance pitfalls which originally motivated us adding it.

What it doesn't solve is the case where someone is running a medium-to-large installation of Pelias because performance will likely be pretty bad for one and two letter inputs.

I would consider allowing finer control of the configuration so that it would remain enabled.. but would not be used in the case where a focus.point was provided since https://github.com/pelias/api/pull/1323 is providing a means of reducing the hit count for those queries.

Also worth noting that having these two different mechanisms for filtering the address layer is confusing, I would like it if we could combine them or delete one of them.

[missinglink]

I'm going to leave this PR open as DRAFT for now so we can use it for testing, however I don't think I'll merge it as-is, I'd considering addressing the following points before merging:

• Always apply to /v1/search even when config set to false (need to make this obvious to the developer)
• Optionally only disable address_layer_filter when focus.point is not supplied in autocomplete mode.

[orangejulius]

In order to allow us to close this PR and/or finish off this thought, I'd propose the second option listed above as the smallest amount of functionality that is worth it to merge:

• only disable address_layer_filter when focus.point is not supplied in autocomplete mode

We'd still have to test it but this would most likely be a positive improvement with little downside.

Otherwise we are looking at adding at least one, if not several config options (for example, allowing individual control of this filter for search and autocomplete).