Instead of hand-managing and running linters, use golangci-lint.

Along with the golangci-lint defaults,
enable a couple other linters we generally agree with.
See also uber-go/zap#1323 for a similar change.

As a result of this, we can:

- Drop the dependabot for tools
- Run the lint job in parallel with build/test
- Simplify the Makefile

Fixes up the `make lint` target to also check
that all files are properly gofmt-ed,
replaces the deprecated `golint` check with `revive`,
and sets up dependabot to also update tools and actions regularly.

This simplifies the Makefile significantly,
borrowing patterns we've used in other projects.
Namely:

- Set GOBIN to a bin subdirectory
  so that we can `go install` dependencies into it.
- Use a shared TEST_FLAGS for `make test` and `make cover`.
  Without this, we're not running with data race detection in CI.
- Build lint step out of separate golint and staticcheck steps.
  In the future, a gofmt step may also be added.
- Move tools dependencies into an unpublished subpackage.

Note: I didn't mess with the 'clean' and 'run' targets
at the bottom of the file even though they're not necessary
to avoid a merge conflict with #60.

This was all added back when these tests were running on Travis CI
and attempted to create an isolated build of sally.

This is all no longer necessary because GitHub Actions and Go modules
provide enough isolation.

Update all dependencies,
use Go 1.18 to build and test,
and delete outdated `bindata` clause in Makefile
(we switched to `go:embed` in #50).

Other minor changes:

- Fix the golint import path
  (it switched to golang.org/x/lint long before it was deprecated)
  and fix the failing lint check.
  (Switching from golint to revive is out of scope for this PR.)
- Delete .envrc -- this was transitional for when Go modules were
  optional.

Go 1.16 includes a `//go:embed` directive that obviates the need for
bindata. Use `//go:embed` instead of bindata for templates used in
sally.

Signed-off-by: Luciano Nooijen <luciano@bytecode.nl>
Co-authored-by: Abhinav Gupta <abg@uber.com>

Set up CI with GitHub actions.

This uses go-bindata/go-bindata to bundle the template in the binary.

I also realized that the handler test was now broken because we were
checking the exact contents of the index page in the test. This change
fixes that too.

This commit modernizes the repository organization a bit.

In short, this,

- Upgrades to Go 1.11.
- Uses Go modules instead of Glide.
- Drops errcheck, which we rarely use in our codebase internally.
- Uses `./...` to run build, tests, etc. since that does not have to
  account for vendor anymore.
- Drops `-installsuffix cgo` from the production build. This was a
  workaround for pre-1.10 versions of Go. See
  https://github.com/golang/go/issues/9344#issuecomment-69944514.
- Uses a tools.go as recommended in [go-modules-by-example/tools][1] to
  pin to tool dependencies.
- Uses `go run` instead of `go install`ing tools globally.

[1]: https://github.com/go-modules-by-example/index/tree/master/010_tools

Tested locally with `make docker-launch`.