add button to reset zoom on overview map

[renaemyers]

Resolves USEPA/TADAShiny#86

[codecov-commenter]

Codecov Report

Merging #355 (3b0890c) into develop (b025c2e) will increase coverage by 0.04%.
The diff coverage is 0.00%.

❗ Current head 3b0890c differs from pull request most recent head a9266eb. Consider uploading reports for the commit a9266eb to get more accurate results

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@             Coverage Diff             @@
##           develop     #355      +/-   ##
===========================================
+ Coverage    53.38%   53.43%   +0.04%     
===========================================
  Files           12       12              
  Lines         2986     2987       +1     
===========================================
+ Hits          1594     1596       +2     
+ Misses        1392     1391       -1     

Files	Coverage Δ
R/Figures.R	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!