feat: implement a feature for previewing posts and single pages

[guqing]

What type of PR is this?

/kind feature
/area core
/area console
/milestone 2.6.x

What this PR does / why we need it:

新增文章和自定义页面预览功能

提供了以下两个路由用于预览，必须登录且是对应文章或自定义页面的 contributors 才能访问，如果不是 contributor 则先得到没有权限访问异常，如果有权限访问但预览文章不存在则得到 404

• 文章预览 GET /preview/posts/{name}
• 自定义页面预览 GET /preview/singlepages/{name}

两个路由都可以通过查询参数 snapshotName 来指定需要预览的内容快照，它是可选的，默认为当前正在编辑的内容

Which issue(s) this PR fixes:

Fixes #2349

Does this PR introduce a user-facing change?

新增文章和自定义页面预览功能

[codecov]

Codecov Report

Merging #3983 (ec09a3d) into main (533f0cf) will decrease coverage by 0.18%.
The diff coverage is 61.11%.

@@             Coverage Diff              @@
##               main    #3983      +/-   ##
============================================
- Coverage     60.52%   60.35%   -0.18%     
- Complexity     2342     2360      +18     
============================================
  Files           353      356       +3     
  Lines         12106    12215     +109     
  Branches        881      889       +8     
============================================
+ Hits           7327     7372      +45     
- Misses         4347     4403      +56     
- Partials        432      440       +8     

Impacted Files	Coverage Δ
...pp/theme/dialect/ContentTemplateHeadProcessor.java	82.35% <ø> (ø)
...app/theme/dialect/TemplateGlobalHeadProcessor.java	94.44% <ø> (ø)
...ain/java/run/halo/app/theme/router/ModelConst.java	100.00% <ø> (ø)
...pp/theme/router/factories/ArchiveRouteFactory.java	74.28% <ø> (ø)
...heme/router/factories/AuthorPostsRouteFactory.java	82.60% <ø> (ø)
...theme/router/factories/CategoriesRouteFactory.java	100.00% <ø> (ø)
...eme/router/factories/CategoryPostRouteFactory.java	15.62% <ø> (ø)
.../app/theme/router/factories/IndexRouteFactory.java	100.00% <ø> (ø)
.../halo/app/theme/router/factories/RouteFactory.java	100.00% <ø> (ø)
...o/app/theme/router/factories/TagsRouteFactory.java	62.50% <ø> (ø)
... and 7 more

... and 11 files with indirect coverage changes

[ruibaby]

测试发现未发布的文章预览时，部分数据没有会导致页面渲染异常，比较常见的是 spec.publishTime，status.lastModifyTime，但仅限于对其做了特殊处理（比如格式化）会导致异常。是否需要对这些字段设置一个 mock 值？

[guqing]

测试发现未发布的文章预览时，部分数据没有会导致页面渲染异常，比较常见的是 spec.publishTime，status.lastModifyTime，但仅限于对其做了特殊处理（比如格式化）会导致异常。是否需要对这些字段设置一个 mock 值？

可以

[guqing]

/hold
wait for console adaptation

[ruibaby]

/unhold

Console 已经适配，目前有两个入口可进行预览：

[ruibaby]

/lgtm

[guqing]

能否在新创建时也显示预览按钮而不是点击了保存之后再显示，反正预览时都会保存内容

[ruibaby]

能否在新创建时也显示预览按钮而不是点击了保存之后再显示，反正预览时都会保存内容

已修改。

[guqing]

/lgtm

[f2c-ci-robot]

@guqing: you cannot LGTM your own PR.

In response to this:

/lgtm

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[JohnNiang]

/lgtm

[f2c-ci-robot]

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