Minor fixes for page header plugin

[shashigharti]

• Partial fixes Make minor changes to existing plugins #892

Page Header

• Rename field from Second text to Highlight text
• For type Image left / Text right - Centralise
• For type Image left / Text right - Add bottom margin (the element is too close to the next one)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8c55a8a) 89.07% compared to head (fd04c7a) 89.07%.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #959   +/-   ##
========================================
  Coverage    89.07%   89.07%           
========================================
  Files           92       92           
  Lines         1126     1126           
  Branches        47       47           
========================================
  Hits          1003     1003           
  Misses         121      121           
  Partials         2        2           

Files Changed	Coverage Δ
foundation/okfplugins/header/models.py	100.00% <100.00%> (ø)

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