test(protocol-designer): mock file-saver while using cypress

[IanLondon]

overview

Same deal as #5932 but for PD

Closes #5457

changelog

• mock file-saver for cypress, using webpack
• define Chrome version in Travis (stable) instead of using implicitly-defined one (it's out-of-date)

review requests

• PD should allow you to download files (branch build, or make dev)
• E2E tests should still pass (if running with yarn cypress open, you need to run make -C protocol-designer dev CYPRESS=1. Which is already the case on edge, just implemented differently)

risk assessment

Medium, but easy to check. If I messed something up, PD would not save files.

[shlokamin]

as discussed via slack not sure what's going on with travis but looks good

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (edge@07f7180). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             edge    #5943   +/-   ##
=======================================
  Coverage        ?   78.95%           
=======================================
  Files           ?      195           
  Lines           ?    18727           
  Branches        ?        0           
=======================================
  Hits            ?    14785           
  Misses          ?     3942           
  Partials        ?        0           

Impacted Files	Coverage Δ
opentrons/protocols/bundle.py	92.98% <0.00%> (ø)
...ot-server/robot_server/service/json_api/factory.py	100.00% <0.00%> (ø)
opentrons/api/dev_types.py	0.00% <0.00%> (ø)
opentrons/legacy_api/robot/mover.py	88.63% <0.00%> (ø)
opentrons/protocol_api/geometry.py	96.17% <0.00%> (ø)
opentrons/drivers/rpi_drivers/types.py	82.92% <0.00%> (ø)
opentrons/system/smoothie_update.py	50.00% <0.00%> (ø)
opentrons/legacy_api/robot/__init__.py	100.00% <0.00%> (ø)
opentrons/drivers/thermocycler/driver.py	49.86% <0.00%> (ø)
opentrons/drivers/serial_communication.py	28.57% <0.00%> (ø)
... and 185 more

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[IanLondon]

I can't figure out what version of Chrome was installed in Travis before, but it didn't have Blob.text fn (so the default Chrome installed was at least ~1 year old??), this kept failing the E2E tests with "text is not a function" (Blob.text fn is used in the Cypress test itself, not used in PD app code)