add basic update functionality

If enabled, jfa-go pings buildrone (hosted at builds.hrfee.pw) every 30
min for new updates. If there is one, it gets information (and if
applicable, a binary) from the appropriate source (buildrone, github, or
dockerhub) and displays it on the admin page. You can switch update
channels between stable and unstable. For binary releases, updates are
downloaded automatically and installed when the user presses update.

Since this obviously introduces some "phone-home" functionality into
jfa-go, I just want to say IPs are not and will not be logged by
buildrone, although I may later introduce functionality to give a rough
idea of the number of users (again, no IPs stored). The whole thing can
also be turned off in settings.

go.mod

@@ -11,6 +11,7 @@ replace github.com/hrfee/jfa-go/common => ./common
 replace github.com/hrfee/jfa-go/ombi => ./ombi
 
 require (
+	github.com/cpuguy83/go-md2man/v2 v2.0.0 // indirect
 	github.com/dgrijalva/jwt-go v3.2.0+incompatible
 	github.com/evanw/esbuild v0.8.50 // indirect
 	github.com/fatih/color v1.10.0
@@ -34,6 +35,7 @@ require (
 	github.com/mailgun/mailgun-go/v4 v4.3.0
 	github.com/mailru/easyjson v0.7.7 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
+	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	github.com/smartystreets/goconvey v1.6.4 // indirect
 	github.com/swaggo/files v0.0.0-20190704085106-630677cd5c14
 	github.com/swaggo/gin-swagger v1.3.0
@@ -41,7 +43,8 @@ require (
 	github.com/ugorji/go v1.2.0 // indirect
 	github.com/writeas/go-strip-markdown v2.0.1+incompatible
 	golang.org/x/crypto v0.0.0-20201112155050-0c6587e931a9 // indirect
-	golang.org/x/sys v0.0.0-20210220050731-9a76102bfb43 // indirect
+	golang.org/x/net v0.0.0-20210226172049-e18ecbb05110 // indirect
+	golang.org/x/sys v0.0.0-20210303074136-134d130e1a04 // indirect
 	golang.org/x/tools v0.1.0 // indirect
 	google.golang.org/protobuf v1.25.0 // indirect
 	gopkg.in/ini.v1 v1.62.0