Fix menu on 'import dashboards' page

[mistercrunch]

Minor bug fix on legacy MVC page
closes #5576

Also minor cosmetic changes:

[codecov-io]

Codecov Report

Merging #5623 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5623      +/-   ##
==========================================
+ Coverage   63.48%   63.48%   +<.01%     
==========================================
  Files         360      360              
  Lines       22886    22889       +3     
  Branches     2547     2547              
==========================================
+ Hits        14529    14531       +2     
- Misses       8342     8343       +1     
  Partials       15       15

Impacted Files	Coverage Δ
superset/models/core.py	85.09% <0%> (-0.1%)	⬇️
superset/viz.py	81.16% <0%> (-0.02%)	⬇️
superset/connectors/druid/models.py	82.56% <0%> (+0.02%)	⬆️

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[mistercrunch]

@timifasubaa @betodealmeida

[betodealmeida]

LGTM