Allow configuring a range for the account validity startup job

[babolivier]

When enabling the account validity feature, Synapse will look at startup for registered account without an expiration date, and will set one equals to now + validity_period for them. On large servers, it can mean that a large number of users will have the same expiration date, which means that they will all be sent a renewal email at the same time, which isn't ideal.

In order to mitigate this, this PR allows server admins to define a max_delta so that the expiration date is a random value in the [now + validity_period ; now + validity_period + max_delta] range. This allows renewal emails to be progressively sent over a configured period instead of being sent all in one big batch.

[codecov]

Codecov Report

Merging #5276 into develop will decrease coverage by 0.56%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #5276      +/-   ##
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- Coverage    62.78%   62.22%   -0.57%     
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  Files          341      336       -5     
  Lines        35452    34712     -740     
  Branches      5798     5688     -110     
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- Hits         22258    21598     -660     
+ Misses       11628    11577      -51     
+ Partials      1566     1537      -29

[erikjohnston]

Rather than having another config option can we just smear the renewal time by, say, 10%? I.e. instead of sending an email exactly 1 week before expiry send it between 1w ± 8h. This has a couple of advantages:

1. It means that the expiry time is always still exact.
2. Doesn't require another config option to be set, and so the account validity feature will always "do the right thing"

[erikjohnston]

IRL: @babolivier is going to look if doing the above is easy, if it is then we'll do that instead, if not we'll do this. Either way, removing review requested for now.

[erikjohnston]

I think we should have it -10% rather than +10%. I think from a security PoV its better to expire earlier rather than later. I.e. setting account expiry at 6w means that all accounts will expire in 6w

Otherwise LGTM.

[babolivier]

Oh, good point