Update dependencies to newest major versions

If I run npm outdated on the current master branch, NPM lists the following packages as having newer major version releases:

Package          Current  Wanted  Latest  Location
@fluent/bundle    0.13.0  0.13.0  0.16.0  send
@fluent/langneg    0.3.0   0.3.0   0.5.0  send
cldr-core         35.1.0  35.1.0  37.0.0  send
configstore        5.0.0     git     git  send
convict            5.2.0   5.2.0   6.0.0  send
helmet            3.23.3  3.23.3   4.1.1  send
mkdirp             0.5.5   0.5.5   1.0.4  send
mozlog             2.2.0   2.2.0   3.0.1  send
redis              2.8.0   2.8.0   3.0.2  send

I think it would be useful to add something like Dependabot to the repository for automatically fetching new package versions and their respective changelogs so it's easier to check if any of the breaking changes between major version releases affect Send.