use unix timestamp for inv created & usedBy

usedBy is still stored as a string in invites.json to cope with existing
invites with times stored formatted. knz/strtime requires cgo for
strptime, so it has been replaced with the native itchyny/timefmt-go.

go.sum

@@ -129,6 +129,8 @@ github.com/gopherjs/gopherjs v0.0.0-20181017120253-0766667cb4d1 h1:EGx4pi6eqNxGa
 github.com/gopherjs/gopherjs v0.0.0-20181017120253-0766667cb4d1/go.mod h1:wJfORRmW1u3UXTncJ5qlYoELFm8eSnnEO6hX4iZ3EWY=
 github.com/hrfee/mediabrowser v0.3.3 h1:7E05uiol8hh2ytKn3WVLrUIvHAyifYEIy3Y5qtuNh8I=
 github.com/hrfee/mediabrowser v0.3.3/go.mod h1:PnHZbdxmbv1wCVdAQyM7nwPwpVj9fdKx2EcET7sAk+U=
+github.com/itchyny/timefmt-go v0.1.2 h1:q0Xa4P5it6K6D7ISsbLAMwx1PnWlixDcJL6/sFs93Hs=
+github.com/itchyny/timefmt-go v0.1.2/go.mod h1:0osSSCQSASBJMsIZnhAaF1C2fCBTJZXrnj37mG8/c+A=
 github.com/jordan-wright/email v4.0.1-0.20200917010138-e1c00e156980+incompatible h1:CL0ooBNfbNyJTJATno+m0h+zM5bW6v7fKlboKUGP/dI=
 github.com/jordan-wright/email v4.0.1-0.20200917010138-e1c00e156980+incompatible/go.mod h1:1c7szIrayyPPB/987hsnvNzLushdWf4o/79s3P08L8A=
 github.com/josharian/intern v1.0.0 h1:vlS4z54oSdjm0bgjRigI+G1HpF+tI+9rE5LLzOg8HmY=
@@ -141,8 +143,6 @@ github.com/json-iterator/go v1.1.10 h1:Kz6Cvnvv2wGdaG/V8yMvfkmNiXq9Ya2KUv4rouJJr
 github.com/json-iterator/go v1.1.10/go.mod h1:KdQUCv79m/52Kvf8AW2vK1V8akMuk1QjK/uOdHXbAo4=
 github.com/jtolds/gls v4.20.0+incompatible h1:xdiiI2gbIgH/gLH7ADydsJ1uDOEzR8yvV7C0MuV77Wo=
 github.com/jtolds/gls v4.20.0+incompatible/go.mod h1:QJZ7F/aHp+rZTRtaJ1ow/lLfFfVYBRgL+9YlvaHOwJU=
-github.com/knz/strtime v0.0.0-20200924090105-187c67f2bf5e h1:ViPE0JEOvtw5I0EGUiFSr2VNKGNU+3oBT+oHbDXHbxk=
-github.com/knz/strtime v0.0.0-20200924090105-187c67f2bf5e/go.mod h1:4ZxfWkxwtc7dBeifERVVWRy9F9rTU9p0yCDgeCtlius=
 github.com/kr/pretty v0.1.0 h1:L/CwN0zerZDmRFUapSPitk6f+Q3+0za1rQkzVuMiMFI=
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=