add localised time and utc time on tooltip

[pindge]

fixes #419, #420

Scope

• add tooltip for displaying time in UTC equiv
• make search page for datasets display in local time
• add flask app config for CUBEDASH_DEFAULT_TIMEZONE
• add CUBEDASH_DEFAULT_TIMEZONE config to README
• make center_time_from_metadata filter to always return time in default_utc
• Change column name Time to Time (local) for clarity
• add test case for checking both localised time and UTC time on tooltip

build fix

latest flask library is broken, pin to 2.1.3

Visual change

[codecov]

Codecov Report

Merging #420 (727bc28) into develop (9ed9bcc) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #420      +/-   ##
===========================================
+ Coverage    86.92%   86.94%   +0.02%     
===========================================
  Files           25       25              
  Lines         3158     3164       +6     
===========================================
+ Hits          2745     2751       +6     
  Misses         413      413              

Impacted Files	Coverage Δ
cubedash/_filters.py	89.37% <100.00%> (+0.34%)	⬆️
cubedash/_model.py	94.16% <100.00%> (+0.04%)	⬆️
cubedash/_utils.py	87.15% <100.00%> (ø)

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[robbibt]

This looks amazing @pindge, very exciting!