Alter issue/comment table TEXT fields to LONGTEXT

[wxiaoguang]

The content field in issue, the content and patch field in comment are all TEXT type.

When using MySQL, TEXT type has a limitation of 64KB chars. It will case failure when migrating a repository containing an issue with very long content.

And, no other database has such limitation as MySQL. So it's good to make issue system's behavior the same with different databases.

Issue: #16656

[wxiaoguang]

The CI failure is caused by TestRepository_GetTag, it seems that it's not related to this PR ......

2021/08/21 14:38:37 ...dules/git/command.go:143:RunWithContext() [D] /drone/src/modules/git/tests/repos/repo1_TestRepository_GetTag: /usr/bin/git -c credential.helper= -c protocol.version=2 show-ref --tags -- lightweightTag
210s
2211 --- FAIL: TestRepository_GetTag (0.21s)
210s
2212 panic: runtime error: invalid memory address or nil pointer dereference [recovered]
210s
2213	panic: runtime error: invalid memory address or nil pointer dereference
210s
2214 [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xac2d5d]
210s
2215
210s
2216 goroutine 1964 [running]:
210s
2217 testing.tRunner.func1.2({0xc170a0, 0x1dd90c0})
210s
2218	/usr/local/go/src/testing/testing.go:1209 +0x36c
210s
2219 testing.tRunner.func1()
210s
2220	/usr/local/go/src/testing/testing.go:1212 +0x3b6
210s
2221 panic({0xc170a0, 0x1dd90c0})
210s
2222	/usr/local/go/src/runtime/panic.go:1047 +0x266
210s
2223 code.gitea.io/gitea/modules/git.TestRepository_GetTag(0x0)
210s
2224	/drone/src/modules/git/repo_tag_test.go:52 +0x29d
210s
2225 testing.tRunner(0xc0016801a0, 0xdcf750)
210s
2226	/usr/local/go/src/testing/testing.go:1259 +0x230
210s
2227 created by testing.(*T).Run
210s
2228	/usr/local/go/src/testing/testing.go:1306 +0x727
210s
2229 FAIL	code.gitea.io/gitea/modules/git	6.324s

[codecov-commenter]

Codecov Report

Merging #16765 (5bd244c) into main (7f85610) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #16765      +/-   ##
==========================================
- Coverage   45.50%   45.49%   -0.02%     
==========================================
  Files         762      762              
  Lines       85939    85939              
==========================================
- Hits        39108    39098      -10     
- Misses      40519    40527       +8     
- Partials     6312     6314       +2     

Impacted Files	Coverage Δ
models/issue.go	58.52% <ø> (ø)
models/issue_comment.go	52.05% <ø> (+0.29%)	⬆️
modules/notification/mail/mail.go	35.29% <0.00%> (-2.95%)	⬇️
models/unit.go	41.09% <0.00%> (-2.74%)	⬇️
modules/process/manager.go	72.83% <0.00%> (-2.47%)	⬇️
modules/log/event.go	60.64% <0.00%> (-1.86%)	⬇️
modules/notification/ui/ui.go	60.86% <0.00%> (-1.45%)	⬇️
models/notification.go	65.13% <0.00%> (-0.88%)	⬇️
services/pull/pull.go	41.78% <0.00%> (ø)
... and 3 more

---

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[zeripath]

Small nit but otherwise LGTM