fix: download log file directly

[chrisleekr]

Description

This fix is to make log files to be downloaded directly from the web server rather than using blob.

Related Issue

#537

Motivation and Context

If the log file is enormous, the export function throws an Invalid string length error.
To avoid the error, create the log file in the public folder and download it directly from the web server.
To avoid out of the disk space, it will only keep the last 10 log files.

How Has This Been Tested?

Screenshots (if appropriate):

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (3d12ad7) compared to base (b341b31).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #561   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           92        92           
  Lines         3400      3419   +19     
  Branches       571       574    +3     
=========================================
+ Hits          3400      3419   +19     

Impacted Files	Coverage Δ
...ntend/webserver/handlers/grid-trade-logs-export.js	100.00% <100.00%> (ø)
app/server-frontend.js	100.00% <100.00%> (ø)

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