Update timeline.endDate when adding new milestone

[maxceem]

Recently, we removed validation of milestone.endDate < timeline.endDate, when adding a new milestone to the timeline.

Now, we have to implement functionality to update timeline's endDate accordingly, when we are adding a new milestone. It's already done when we update milestones, see https://github.com/topcoder-platform/tc-project-service/blob/dev/src/routes/milestones/update.js#L301, so we can implement it a similar way.

Note, that there is some issues in this functionality for updating regarding updating timeline in ES, see issue #376

[maxceem]

I think we can skip this issue for now, as we are going to reformat the format of milestones to points, and we wouldn't have such functionality, see the related issue appirio-tech/connect-app#3299.

[vikasrohit]

Lets put this on hold for some time. Will close it once we wrap appirio-tech/connect-app#3299