#1170: upgrade typescript version to 2.6.2 (closes #1170)

[giogonzo]

Closes #1170

Test Plan

test build and start look 👍
some more context on the TS4023 error: microsoft/TypeScript#5711
and on the "Arbitrary expression.." one: https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#arbitrary-expressions-are-forbidden-in-export-assignments-in-ambient-contexts

tests performed

A Test Plan is used to show what you tested to make sure your code works fine. You should write here all that you did to test, and provide some results of your testing.

These results can be screenshots, query results, or even just pointers to unit tests that successfully passed.

cross browser compatibility

Should be compatible with every browser below. Check the ones you tested!

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tests not performed (domain coverage)

At times not everything can be tested, and writing what hasn't been tested is just as important as writing what has been tested.

An example of partial test is a field displaying 4 possible values. If 3 values are tested, with screenshots, and 1 is not, then it should be mentioned here.

[FrancescoCioria]

(I think we should deeply test Tablo typings as soon as we have time/the change)

[nemobot]