Link to previous blames in file blame page (#16259)

Adds a link to each blame hunk, to view the blame of an earlier version of the file, similar to GitHub. Also refactors the blame render from fmtstring based to template based.

* Fix blame bottom line and add blame prior button

* Jump to previous parent commit from the commit.

* Fix previous commit link

* Fix previous blame link

* Fix the given file not exist in the previous commit.

* Fix blameRow struct not export

* fix theming issues, rename template var

* remove unused LastCommit fetch

* fix location of blame-hunk divider

* rewrite previous commit checks

* remove duplicate commit lookup

its already resolved and stored in ctx.Repo.Commit!

* split out blamePart processing into function

Co-authored-by: rogerluo410 <rogerluo410@gmail.com>

web_src/less/_base.less

@@ -106,6 +106,7 @@
   --color-markup-code-block: #00000010;
   --color-button: #ffffff;
   --color-code-bg: #ffffff;
+  --color-code-sidebar-bg: #f5f5f5;
   --color-shadow: #00000030;
   --color-secondary-bg: #f4f4f4;
   --color-expand-button: #d8efff;
@@ -1442,6 +1443,14 @@ a.ui.label:hover {
   margin-right: 0;
 }
 
+.lines-blame-btn {
+  padding-left: 10px;
+  padding-right: 10px;
+  text-align: right !important;
+  background-color: var(--color-code-sidebar-bg);
+  width: 2%;
+}
+
 .lines-num {
   padding-left: 10px;
   padding-right: 10px;
@@ -1507,7 +1516,7 @@ a.ui.label:hover {
 
 .blame .lines-num {
   padding: 0 !important;
-  background-color: #f5f5f5;
+  background-color: var(--color-code-sidebar-bg);
 }
 
 .blame .lines-code {
@@ -1532,7 +1541,7 @@ a.ui.label:hover {
   vertical-align: top;
   color: #999999;
   padding: 0 !important;
-  background: #f5f5f5;
+  background: var(--color-code-sidebar-bg);
   width: 1%;
   -moz-user-select: none;
   -ms-user-select: none;
@@ -1574,6 +1583,10 @@ a.ui.label:hover {
   }
 }
 
+.top-line-blame {
+  border-top: 1px solid var(--color-secondary);
+}
+
 .lines-code,
 .lines-commit {
   .bottom-line {