[openstreetmap/openstreetmap-website] Fix test suite to work in non-UK timezones (PR #4063)

[Andy Allan]

The test suit currently fails if your development environment (e.g. your own laptop) is set to a timezone outside of the UK. I encountered this earlier this year, at the Karlsruhe Hack Weekend, but I'm surprised more people haven't complained about it! The failing test is:

```
Failure:
ApplicationHelperTest#test_friendly_date [/home/andy/src/openstreetmap-website/test/helpers/application_helper_test.rb:62]:
Expected /^<span title=" *5 March 2014 at 18:58">.*<\/span>$/ to match "<span title=\" 5 March 2014 at 17:58\">over 9 years</span>".
```

This is because that test uses `Time.new(...).utc`, which is not the same thing as `Time.utc(...)`.

But why does it work - all year round - in the UK? 

`Time.new` takes an optional list of year, month, day etc (defaults to midnight first of January) and an implicit "in the current system timezone". If you ask for a date in the winter, it will use the winter equivalent of your current timezone. For example:

```ruby
>> Time.now.zone
=> "BST"
>> Time.new(2020).zone
=> "GMT"
```

Therefore, there is no problem in either summer or winter for the UK, both approaches give the same time. For example:

```ruby
>> Time.new(2020).utc
=> 2020-01-01 00:00:00 UTC
>> Time.utc(2020)
=> 2020-01-01 00:00:00 UTC
```

Imagine the first line above says `Time.new(2020, 'GMT').utc` i.e. midnight GMT. Then convert that to UTC, which is the same time, also midnight.

However, if you switch your laptop to a different timezone:

```ruby
>> Time.now.zone
=> "CEST"
>> Time.new(2020).zone
=> "CET"
>> Time.new(2020).utc
=> 2019-12-31 23:00:00 UTC
>> Time.utc(2020)
=> 2020-01-01 00:00:00 UTC
```

The third line can now be considered as `Time.new(2020, CET).utc` i.e. midnight 1st January in CET, which is then converted to 23:00 the previous day in UTC.

This test previously used `Time.new(2014, 3, 5, 18, 58, 23).utc` which we can see will be created in the local timezone at 18:58 and then converted to some other time in UTC, depending on your timezone offset from UTC e.g. it might be 17:58 UTC if you are in CE(S)T.

We already used two other forms of date expression in the tests, both `Time.new(..., "+00:00")` and also `Time.utc(...)`, neither of which have this problem. I've standardized on `Time.utc(...)` in this PR because it allows the minutes, seconds etc to be omitted when they are unimportant, for better readability.

You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/4063

-- Commit Summary --

  * Fix test to work in non-UK timezones
  * Use Time.utc for consistency with other tests

-- File Changes --

    M test/helpers/application_helper_test.rb (2)
    M test/helpers/note_helper_test.rb (4)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/4063.patch
https://github.com/openstreetmap/openstreetmap-website/pull/4063.diff

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