Clean up polluted styles and remove dead CSS code (#24497)

Follow #24393


The funny history:

* At the beginning, `.ui.message` was polluted by `text-align: center`
* Then people do `<div class="ui ... message text left">`
* But `.ui.left` is polluted by `float: left`
* Then people do `#xxx .ui.message { width: 100% !important;}`

The code just becomes more and more hacky.

After removing the pollution,  everything becomes clear and straight.


And, this PR also does:

1. Remove the `package.css`, its styles could be provided by `top
aligned`
2. Remove `#avatar-arrow`, dead code



Screenshot:


![image](https://user-images.githubusercontent.com/2114189/235862130-a9eb5d8f-7d01-457c-99f7-21d0abc3075e.png)


![image](https://user-images.githubusercontent.com/2114189/235862222-139709a7-95c2-4f89-a40f-100b2d76d9bb.png)

Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/base.css

@@ -1556,7 +1556,7 @@ img.ui.avatar,
   vertical-align: middle;
 }
 
-.ui .message {
+.ui .message.flash-message {
   text-align: center;
 }
 

web_src/css/index.css

@@ -41,6 +41,5 @@
 @import "./admin.css";
 @import "./explore.css";
 @import "./review.css";
-@import "./package.css";
 @import "./runner.css";
 @import "./helpers.css";

web_src/css/package.css

@@ -1,7 +0,0 @@
-.container-labels td:nth-child(1) {
-  vertical-align: top;
-}
-
-.container-labels td:nth-child(2) {
-  overflow-wrap: anywhere;
-}

web_src/css/repository.css

@@ -2600,39 +2600,6 @@
   height: 15px;
 }
 
-#avatar-arrow::before,
-#avatar-arrow::after {
-  right: 100%;
-  top: 20px;
-  border: solid transparent;
-  content: " ";
-  height: 0;
-  width: 0;
-  position: absolute;
-  pointer-events: none;
-}
-
-#avatar-arrow::before {
-  border-right-color: var(--color-secondary);
-  border-width: 9px;
-  margin-top: -9px;
-}
-
-#avatar-arrow::after {
-  border-right-color: var(--color-box-header);
-  border-width: 8px;
-  margin-top: -8px;
-}
-
-#transfer-repo-modal .ui.message,
-#delete-repo-modal .ui.message,
-#delete-wiki-modal .ui.message,
-#convert-fork-repo-modal .ui.message,
-#convert-mirror-repo-modal .ui.message,
-#fork-repo-modal .ui.message {
-  width: 100% !important;
-}
-
 .tab-size-1 {
   tab-size: 1 !important;
   -moz-tab-size: 1 !important;