[$70] Only admins should be able to update actualStartDate and completionDate of milestones

[maxceem]

Currently, all the users can update actualStartDate and completionDate of milestones. We should limit users who can update them, so only user with admin roles can update them.

For checking permissions we can reuse method hasPermission without providing the third argument, like this:

hasPermission({topcoderRoles: ADMIN_ROLES}, req.authUser)

We also have to write at least 4 unit tests which wold validate that:

1. non-admin users cannot update actualStartDate
2. non-admin users cannot update completionDate
3. admin users can update actualStartDate
4. admin users can update completionDate

Ref: appirio-tech/connect-app#3210

[r0hit-gupta]

Please assign me.

[r0hit-gupta]

PR at #382

[r0hit-gupta]

@maxceem sorry for the huge delay. Fixing it ASAP.

[r0hit-gupta]

@maxceem I have updated the PR. Please review now. Thanks.